implement persistent equivalence graph for relationship tracking
Tara L Andrews [Sat, 21 Apr 2012 13:07:49 +0000 (15:07 +0200)]
lib/Text/Tradition/Collation.pm
lib/Text/Tradition/Collation/RelationshipStore.pm
t/data/legendfrag.xml
t/text_tradition_collation_relationshipstore.t

index 2c5e40c..34151eb 100644 (file)
@@ -31,6 +31,8 @@ has 'relations' => (
                related_readings => 'related_readings',
                get_relationship => 'get_relationship',
                del_relationship => 'del_relationship',
+               equivalence => 'equivalence',
+               equivalence_graph => 'equivalence_graph',
        },
        writer => '_set_relations',
        );
@@ -429,10 +431,13 @@ sub add_path {
 
        $self->_graphcalc_done(0);
        # Connect the readings
-    $self->sequence->add_edge( $source, $target );
+       unless( $self->sequence->has_edge( $source, $target ) ) {
+           $self->sequence->add_edge( $source, $target );
+           $self->relations->add_equivalence_edge( $source, $target );
+       }
     # Note the witness in question
     $self->sequence->set_edge_attribute( $source, $target, $wit, 1 );
-};
+}
 
 sub del_path {
        my $self = shift;
@@ -454,6 +459,7 @@ sub del_path {
        }
        unless( keys %{$self->sequence->get_edge_attributes( $source, $target )} ) {
                $self->sequence->delete_edge( $source, $target );
+               $self->relations->delete_equivalence_edge( $source, $target );
        }
 }
 
@@ -1055,7 +1061,6 @@ sub as_graphml {
 
     # Add the path edges to the sequence graph
     my $edge_ctr = 0;
-    $DB::single = 1;
     foreach my $e ( sort { $a->[0] cmp $b->[0] } $self->sequence->edges() ) {
        # We add an edge in the graphml for every witness in $e.
        next unless( $use_readings{$e->[0]} || $use_readings{$e->[1]} );
@@ -1449,69 +1454,6 @@ sub make_witness_path {
     $wit->clear_uncorrected_path;
 }
 
-=head2 equivalence_graph( \%readingmap, $startrank, $endrank, @testvector )
-
-Returns an equivalence graph of the collation, in which all readings
-related via a 'colocated' relationship are transformed into a single
-vertex. Can be used to determine the validity of a new relationship.  The
-mapping between equivalence vertices and reading IDs will be stored in the
-hash whose reference is passed as readingmap. For a subset of the graph,
-pass in a start and/or an ending rank (this only works if L<calculate_ranks>
-has been called at least once.)
-
-It is also possible to pass in a test relationship in @testvector, and get
-the resulting equivalence graph before the relationship has been made.
-
-=cut
-
-sub equivalence_graph {
-       my( $self, $map, $start, $end, @newvector ) = @_;
-       $start = undef unless $self->end->has_rank;
-       $end = undef unless $self->end->has_rank;
-       my $eqgraph = Graph->new();
-    my $rel_ctr = 0;
-    # Add the nodes
-    foreach my $r ( $self->readings ) {
-       unless( $r eq $self->start || $r eq $self->end ) {
-                       next if $start && $r->rank < $start;
-                       next if $end && $r->rank > $end;
-        }
-               next if exists $map->{$r->id};
-        my @rels = $self->related_readings( $r->id, 'colocated' );
-        push( @rels, $r->id );
-               # Make an equivalence vertex
-               my $rn = 'equivalence_' . $rel_ctr++;
-               $eqgraph->add_vertex( $rn );
-               # Note which readings belong to this vertex.
-               push( @rels, $r->id );
-               foreach( @rels ) {
-                       $map->{$_} = $rn;
-               }
-    }
-
-    # Add the edges.
-    foreach my $p ( $self->paths ) {
-       my $efrom = exists $map->{$p->[0]} ? $map->{$p->[0]} 
-               : $map->{$self->start->id};
-       my $eto = exists $map->{$p->[1]} ? $map->{$p->[1]} 
-               : $map->{$self->end->id};
-       $eqgraph->add_edge( $efrom, $eto );
-    }
-    
-    # Collapse the vertices in @newvector if applicable.
-    if( @newvector ) {
-               my( $eqs, $eqt ) = map { $map->{$_} } @newvector;
-               $DB::single = 1 unless $eqs && $eqt;
-               unless( $eqs eq $eqt ) {
-                       # Combine the vertices.
-                       map { $eqgraph->add_edge( $eqs, $_ ) } $eqgraph->successors( $eqt );
-                       map { $eqgraph->add_edge( $_, $eqs ) } $eqgraph->predecessors( $eqt );
-                       $eqgraph->delete_vertex( $eqt );
-               }
-       }
-    return $eqgraph;
-}
-
 =head2 calculate_ranks
 
 Calculate the reading ranks (that is, their aligned positions relative
@@ -1547,28 +1489,25 @@ sub calculate_ranks {
     # Save the existing ranks, in case we need to invalidate the cached SVG.
     my %existing_ranks;
     map { $existing_ranks{$_} = $_->rank } $self->readings;
-    # Walk a version of the graph where every node linked by a relationship 
-    # edge is fundamentally the same node, and do a topological ranking on
-    # the nodes in this graph.
-    my %rel_containers;
-    my $topo_graph = $self->equivalence_graph( \%rel_containers );
-    
-    # Now do the rankings, starting with the start node.
-    my $topo_start = $rel_containers{$self->start->id};
+
+    # Do the rankings based on the relationship equivalence graph, starting 
+    # with the start node.
+    my $topo_start = $self->equivalence( $self->start->id );
     my $node_ranks = { $topo_start => 0 };
     my @curr_origin = ( $topo_start );
     # A little iterative function.
     while( @curr_origin ) {
-        @curr_origin = _assign_rank( $topo_graph, $node_ranks, @curr_origin );
+        @curr_origin = _assign_rank( $self->equivalence_graph, 
+               $node_ranks, @curr_origin );
     }
     # Transfer our rankings from the topological graph to the real one.
     foreach my $r ( $self->readings ) {
-        if( defined $node_ranks->{$rel_containers{$r->id}} ) {
-            $r->rank( $node_ranks->{$rel_containers{$r->id}} );
+        if( defined $node_ranks->{$self->equivalence( $r->id )} ) {
+            $r->rank( $node_ranks->{$self->equivalence( $r->id )} );
         } else {
                # Die. Find the last rank we calculated.
-               my @all_defined = sort { ( $node_ranks->{$rel_containers{$a->id}}||-1 )
-                                <=> ( $node_ranks->{$rel_containers{$b->id}}||-1 ) }
+               my @all_defined = sort { ( $node_ranks->{$self->equivalence( $a->id )}||-1 )
+                                <=> ( $node_ranks->{$self->equivalence( $b->id )}||-1 ) }
                        $self->readings;
                my $last = pop @all_defined;
             throw( "Ranks not calculated after $last - do you have a cycle in the graph?" );
index 3024f20..91a63af 100644 (file)
@@ -85,6 +85,59 @@ has 'graph' => (
     },
        );
        
+=head2 equivalence_graph()
+
+Returns an equivalence graph of the collation, in which all readings
+related via a 'colocated' relationship are transformed into a single
+vertex. Can be used to determine the validity of a new relationship. 
+
+=cut
+
+has 'equivalence_graph' => (
+       is => 'ro',
+       isa => 'Graph',
+       default => sub { Graph->new() },
+       );
+       
+has '_node_equivalences' => (
+       is => 'ro',
+       traits => ['Hash'],
+       handles => {
+               equivalence => 'get',
+               set_equivalence => 'set',
+               remove_equivalence => 'delete',
+       },
+       );
+
+has '_equivalence_readings' => (
+       is => 'ro',
+       traits => ['Hash'],
+       handles => {
+               eqreadings => 'get',
+               set_eqreadings => 'set',
+               remove_eqreadings => 'delete',
+       },
+       );
+       
+around add_reading => sub {
+       my $orig = shift;
+       my $self = shift;
+       
+       $self->equivalence_graph->add_vertex( @_ );
+       $self->set_equivalence( $_[0], $_[0] );
+       $self->set_eqreadings( $_[0], [ $_[0] ] );
+       $self->$orig( @_ );
+};
+
+around delete_reading => sub {
+       my $orig = shift;
+       my $self = shift;
+       
+       $DB::single = 1;
+       $self->_remove_equivalence_node( @_ );
+       $self->$orig( @_ );
+};
+
 =head2 get_relationship
 
 Return the relationship object, if any, that exists between two readings.
@@ -112,6 +165,7 @@ sub _set_relationship {
        my( $self, $relationship, @vector ) = @_;
        $self->graph->add_edge( @vector );
        $self->graph->set_edge_attribute( @vector, 'object', $relationship );
+       $self->make_equivalence( @vector ) if $relationship->colocated;
 }
 
 =head2 create
@@ -231,6 +285,15 @@ try {
        ok( 0, "Collation now has a cycle" );
 }
 
+# Now attempt merge of an identical reading
+try {
+       $c1->merge_readings( '9,3', '11,5' );
+       ok( 1, "Successfully merged reading 'pontifex'" );
+} catch ( Text::Tradition::Error $e ) {
+       ok( 0, "Merge of mergeable readings failed: $e->message" );
+       
+}
+
 # Test 2: try to equate nodes that are prevented with a real intermediate
 # equivalence
 my $t2 = Text::Tradition->new( 'input' => 'Self', 'file' => 't/data/legendfrag.xml' );
@@ -302,6 +365,7 @@ sub add_relationship {
        my( $self, $source, $target, $options ) = @_;
     my $c = $self->collation;
 
+       throw( "Adding self relationship at $source" ) if $source eq $target;
        my $relationship;
        my $thispaironly;
        my $droppedcolls = [];
@@ -449,15 +513,16 @@ sub del_relationship {
        my( $self, $source, $target ) = @_;
        my $rel = $self->get_relationship( $source, $target );
        return () unless $rel; # Nothing to delete; return an empty set.
+       my $colo = $rel->colocated;
        my @vectors = ( [ $source, $target ] );
-       $self->_remove_relationship( $source, $target );
+       $self->_remove_relationship( $colo, $source, $target );
        if( $rel->nonlocal ) {
                # Remove the relationship wherever it occurs.
                # Remove the relationship wherever it occurs.
                my @rel_edges = grep { $self->get_relationship( @$_ ) == $rel }
                        $self->relationships;
                foreach my $re ( @rel_edges ) {
-                       $self->_remove_relationship( @$re );
+                       $self->_remove_relationship( $colo, @$re );
                        push( @vectors, $re );
                }
                $self->del_scoped_relationship( $rel->reading_a, $rel->reading_b );
@@ -466,8 +531,9 @@ sub del_relationship {
 }
 
 sub _remove_relationship {
-       my( $self, @vector ) = @_;
+       my( $self, $equiv, @vector ) = @_;
        $self->graph->delete_edge( @vector );
+       $self->break_equivalence( @vector ) if $equiv;
 }
        
 =head2 relationship_valid( $source, $target, $type )
@@ -502,20 +568,10 @@ sub relationship_valid {
                # We also need to check both that the readings occur in distinct
                # witnesses, and that they are not in the same place. That is,
                # proposing to link them should cause a witness loop.
-               my $map = {};
-               my( $startrank, $endrank );
-               if( $c->end->has_rank ) {
-                       my $cpred = $c->common_predecessor( $source, $target );
-                       my $csucc = $c->common_successor( $source, $target );
-                       $startrank = $cpred->rank;
-                       $endrank = $csucc->rank;
-               }
-               my $eqgraph = $c->equivalence_graph( $map, $startrank, $endrank, 
-                       $source, $target );
-               if( $eqgraph->has_a_cycle ) {
-                       return ( 1, "ok" );
-               } else {
+               if( $self->test_equivalence( $source, $target ) ) {
                        return ( 0, "Readings appear to be colocated, not transposed" );
+               } else {
+                       return ( 1, "ok" );
                }
                
        } elsif( $rel ne 'repetition' ) {
@@ -529,24 +585,16 @@ sub relationship_valid {
                unless( $rel eq 'collated' || $sourcerank == $targetrank ) {
                        push( @$mustdrop, $self->_drop_collations( $source ) );
                        push( @$mustdrop, $self->_drop_collations( $target ) );
-               }
-               my $map = {};
-               my( $startrank, $endrank );
-               if( $c->end->has_rank ) {
-                       my $cpred = $c->common_predecessor( $source, $target );
-                       my $csucc = $c->common_successor( $source, $target );
-                       $startrank = $cpred->rank;
-                       $endrank = $csucc->rank;
-                       unless( $rel eq 'collated' || $sourcerank == $targetrank ) {
-                               foreach my $rk ( $startrank+1 .. $endrank-1 ) {
+                       if( $c->end->has_rank ) {
+                               my $cpred = $c->common_predecessor( $source, $target );
+                               my $csucc = $c->common_successor( $source, $target );
+                               foreach my $rk ( $cpred->rank+1 .. $csucc->rank-1 ) {
                                        map { push( @$mustdrop, $self->_drop_collations( $_->id ) ) }
                                                $c->readings_at_rank( $rk );
                                }
                        }
                }
-               my $eqgraph = $c->equivalence_graph( $map, $startrank, $endrank, 
-                       $source, $target );
-               if( $eqgraph->has_a_cycle ) {
+               unless( $self->test_equivalence( $source, $target ) ) {
                        $self->_restore_collations( @$mustdrop );
                        return( 0, "Relationship would create witness loop" );
                }
@@ -561,6 +609,7 @@ sub _drop_collations {
                if( $self->get_relationship( $reading, $n )->type eq 'collated' ) {
                        push( @dropped, [ $reading, $n ] );
                        $self->del_relationship( $reading, $n );
+                       #print STDERR "Dropped collation $reading -> $n\n";
                }
        }
        return @dropped;
@@ -571,6 +620,7 @@ sub _restore_collations {
        foreach my $v ( @vectors ) {
                try {
                        $self->add_relationship( @$v, { 'type' => 'collated' } );
+                       #print STDERR "Restored collation @$v\n";
                } catch {
                        print STDERR $v->[0] . " - " . $v->[1] . " no longer collate\n";
                }
@@ -686,9 +736,260 @@ sub merge_readings {
                $rel = $self->get_relationship( @$edge );
                $self->_set_relationship( $rel, @vector );
        }
-       $self->delete_reading( $deleted );
+       $self->make_equivalence( $deleted, $kept );
 }
 
+### Equivalence logic
+
+sub _remove_equivalence_node {
+       my( $self, $node ) = @_;
+       my $group = $self->equivalence( $node );
+       my $nodelist = $self->eqreadings( $group );
+       if( @$nodelist == 1 && $nodelist->[0] eq $node ) {
+               #print STDERR "Removing equivalence $group for $node\n";
+               $self->remove_eqreadings( $group );
+       } elsif( @$nodelist == 1 ) {
+               warn "DATA INCONSISTENCY in equivalence graph: " . $nodelist->[0] .
+                       " in group that should have only $node";
+       } else {
+               #print STDERR "Removing $node from equivalence $group\n";
+               my @newlist = grep { $_ ne $node } @$nodelist;
+               $self->set_eqreadings( $group, \@newlist );
+               $self->remove_equivalence( $node );
+       }
+}
+
+=head2 add_equivalence_edge
+
+Return the relationship object, if any, that exists between two readings.
+
+=cut
+
+sub add_equivalence_edge {
+       my( $self, $source, $target ) = @_;
+       my $seq = $self->equivalence( $source );
+       my $teq = $self->equivalence( $target );
+       #print STDERR "Adding equivalence edge $seq -> $teq for $source -> $target\n";
+       $self->equivalence_graph->add_edge( $seq, $teq );
+}
+
+=head2 add_equivalence_edge
+
+Return the relationship object, if any, that exists between two readings.
+
+=cut
+
+sub delete_equivalence_edge {
+       my( $self, $source, $target ) = @_;
+       my $seq = $self->equivalence( $source );
+       my $teq = $self->equivalence( $target );
+       #print STDERR "Deleting equivalence edge $seq -> $teq for $source -> $target\n";
+       $self->equivalence_graph->delete_edge( $seq, $teq );
+}
+
+sub _is_disconnected {
+       my $self = shift;
+       return( scalar $self->equivalence_graph->predecessorless_vertices > 1
+               || scalar $self->equivalence_graph->successorless_vertices > 1 );
+}
+
+=head2 make_equivalence
+
+Equate two readings in the equivalence graph.  Should only be called internally.
+
+=cut
+
+sub make_equivalence {
+       my( $self, $source, $target ) = @_;
+       # Get the source equivalent readings
+       my $seq = $self->equivalence( $source );
+       my $teq = $self->equivalence( $target );
+       # Nothing to do if they are already equivalent...
+       return if $seq eq $teq;
+       #print STDERR "Making equivalence for $source -> $target\n";
+       my $sourcepool = $self->eqreadings( $seq );
+       # and add them to the target readings.
+       # print STDERR "Moving readings '@$sourcepool' from group $seq to $teq\n";
+       push( @{$self->eqreadings( $teq )}, @$sourcepool );
+       map { $self->set_equivalence( $_, $teq ) } @$sourcepool;
+       # Then merge the nodes in the equivalence graph.
+       foreach my $pred ( $self->equivalence_graph->predecessors( $seq ) ) {
+               $self->equivalence_graph->add_edge( $pred, $teq );
+       }
+       foreach my $succ ( $self->equivalence_graph->successors( $seq ) ) {
+               $self->equivalence_graph->add_edge( $teq, $succ );
+       }
+       $self->equivalence_graph->delete_vertex( $seq );
+#      throw( "Graph got disconnected making $source / $target equivalence" )
+#              if $self->_is_disconnected;
+}
+
+=head2 test_equivalence
+
+Test whether, if two readings were equated with a relationship, the graph would
+still be valid.
+
+=cut
+
+sub test_equivalence {
+       my( $self, $source, $target ) = @_;
+       # Try merging the nodes in the equivalence graph; return a true value if
+       # no cycle is introduced thereby. Restore the original graph first.
+       
+       # Keep track of edges we add
+       my %added_pred;
+       my %added_succ;
+       # Get the reading equivalents
+       my $seq = $self->equivalence( $source );
+       my $teq = $self->equivalence( $target );
+       # Maybe this is easy?
+       return 1 if $seq eq $teq;
+       
+       # Save the first graph
+       my $checkstr = $self->equivalence_graph->stringify();
+       # Add and save relevant edges
+       foreach my $pred ( $self->equivalence_graph->predecessors( $seq ) ) {
+               if( $self->equivalence_graph->has_edge( $pred, $teq ) ) {
+                       $added_pred{$pred} = 0;
+               } else {
+                       $self->equivalence_graph->add_edge( $pred, $teq );
+                       $added_pred{$pred} = 1;
+               }
+       }
+       foreach my $succ ( $self->equivalence_graph->successors( $seq ) ) {
+               if( $self->equivalence_graph->has_edge( $teq, $succ ) ) {
+                       $added_succ{$succ} = 0;
+               } else {
+                       $self->equivalence_graph->add_edge( $teq, $succ );
+                       $added_succ{$succ} = 1;
+               }
+       }
+       # Delete source equivalent and test
+       $self->equivalence_graph->delete_vertex( $seq );
+       my $ret = !$self->equivalence_graph->has_a_cycle;
+       
+       # Restore what we changed
+       $self->equivalence_graph->add_vertex( $seq );
+       foreach my $pred ( keys %added_pred ) {
+               $self->equivalence_graph->add_edge( $pred, $seq );
+               $self->equivalence_graph->delete_edge( $pred, $teq ) if $added_pred{$pred};
+       }
+       foreach my $succ ( keys %added_succ ) {
+               $self->equivalence_graph->add_edge( $seq, $succ );
+               $self->equivalence_graph->delete_edge( $teq, $succ ) if $added_succ{$succ};
+       }
+       unless( $self->equivalence_graph->eq( $checkstr ) ) {
+               warn "GRAPH CHANGED after testing";
+       }
+       # Return our answer
+       return $ret;
+}
+
+=head2 break_equivalence
+
+Unmake an equivalence link between two readings. Should only be called internally.
+
+=cut
+
+sub break_equivalence {
+       my( $self, $source, $target ) = @_;
+       
+       # This is the hard one. Need to reconstruct the equivalence groups without
+       # the given link.
+       my( %sng, %tng );
+       map { $sng{$_} = 1 } $self->_find_equiv_without( $source, $target );
+       map { $tng{$_} = 1 } $self->_find_equiv_without( $target, $source );
+       # If these groups intersect, they are still connected; do nothing.
+       foreach my $el ( keys %tng ) {
+               if( exists $sng{$el} ) {
+                       #print STDERR "Equivalence break $source / $target is a noop\n";
+                       return;
+               }
+       }
+       #print STDERR "Breaking equivalence $source / $target\n";
+       # If they don't intersect, then we split the nodes in the graph and in
+       # the hashes. First figure out which group has which name
+       my $oldgroup = $self->equivalence( $source ); # eq for $target
+       my $swapped = $oldgroup eq $source;
+       my $newgroup = $swapped ? $target : $source;
+       my( $oldmembers, $newmembers );
+       if( $swapped ) {
+               $oldmembers = [ keys %sng ];
+               $newmembers = [ keys %tng ];
+       } else {
+               $oldmembers = [ keys %tng ];
+               $newmembers = [ keys %sng ];
+       }
+               
+       # First alter the old group in the hash
+       $self->set_eqreadings( $oldgroup, $oldmembers );
+       
+       # then add the new group back to the hash with its new key
+       $self->set_eqreadings( $newgroup, $newmembers );
+       foreach my $el ( @$newmembers ) {
+               $self->set_equivalence( $el, $newgroup );
+       }
+       
+       # Now add the new group back to the equivalence graph
+       $self->equivalence_graph->add_vertex( $newgroup );
+       # ...add the appropriate edges to the source group vertext
+       my $c = $self->collation;
+       foreach my $rdg ( @$newmembers ) {
+               foreach my $rp ( $c->sequence->predecessors( $rdg ) ) {
+                       $self->equivalence_graph->add_edge( $self->equivalence( $rp ), $newgroup );
+               }
+               foreach my $rs ( $c->sequence->successors( $rdg ) ) {
+                       $self->equivalence_graph->add_edge( $newgroup, $self->equivalence( $rs ) );
+               }
+       }
+       
+       # ...and figure out which edges on the old group vertex to delete.
+       my( %old_pred, %old_succ );
+       foreach my $rdg ( @$oldmembers ) {
+               foreach my $rp ( $c->sequence->predecessors( $rdg ) ) {
+                       $old_pred{$self->equivalence( $rp )} = 1;
+               }
+               foreach my $rs ( $c->sequence->successors( $rdg ) ) {
+                       $old_succ{$self->equivalence( $rs )} = 1;
+               }
+       }
+       foreach my $p ( $self->equivalence_graph->predecessors( $oldgroup ) ) {
+               unless( $old_pred{$p} ) {
+                       $self->equivalence_graph->delete_edge( $p, $oldgroup );
+               }
+       }
+       foreach my $s ( $self->equivalence_graph->successors( $oldgroup ) ) {
+               unless( $old_succ{$s} ) {
+                       $self->equivalence_graph->delete_edge( $oldgroup, $s );
+               }
+       }
+#      throw( "Graph got disconnected breaking $source / $target equivalence" )
+#              if $self->_is_disconnected;
+}
+
+sub _find_equiv_without {
+       my( $self, $first, $second ) = @_;
+       my %found = ( $first => 1 );
+       my $check = [ $first ];
+       my $iter = 0;
+       while( @$check ) {
+               my $more = [];
+               foreach my $r ( @$check ) {
+                       foreach my $nr ( $self->graph->neighbors( $r ) ) {
+                               next if $r eq $second;
+                               if( $self->get_relationship( $r, $nr )->colocated ) {
+                                       push( @$more, $nr ) unless exists $found{$nr};
+                                       $found{$nr} = 1;
+                               }
+                       }
+               }
+               $check = $more;
+       }
+       return keys %found;
+}
+
+### Output logic
+
 sub _as_graphml { 
        my( $self, $graphml_ns, $xmlroot, $node_hash, $nodeid_key, $edge_keys ) = @_;
        
index abb15d5..960a948 100644 (file)
       <data key="de8">collated</data>
       <data key="de7">local</data>
     </edge>
-    <edge source="n29" target="n29" id="e13">
-      <data key="de5">pontifex</data>
-      <data key="de6">pontifex</data>
-      <data key="de8">collated</data>
-      <data key="de7">local</data>
-    </edge>
   </graph>
 </graphml>
index d0c3883..7b8e989 100644 (file)
@@ -69,6 +69,15 @@ try {
        ok( 0, "Collation now has a cycle" );
 }
 
+# Now attempt merge of an identical reading
+try {
+       $c1->merge_readings( '9,3', '11,5' );
+       ok( 1, "Successfully merged reading 'pontifex'" );
+} catch ( Text::Tradition::Error $e ) {
+       ok( 0, "Merge of mergeable readings failed: $e->message" );
+       
+}
+
 # Test 2: try to equate nodes that are prevented with a real intermediate
 # equivalence
 my $t2 = Text::Tradition->new( 'input' => 'Self', 'file' => 't/data/legendfrag.xml' );