prevent merge_readings from causing witness loops

diff --git a/base/lib/Text/Tradition/Collation.pm b/base/lib/Text/Tradition/Collation.pm
index 904d39d..b4489f2 100644 (file)
--- a/base/lib/Text/Tradition/Collation.pm
+++ b/base/lib/Text/Tradition/Collation.pm
@@ -357,6 +357,22 @@ sub merge_readings {
                                || $del_obj eq $self->start || $del_obj eq $self->end );
                throw( "Cannot combine text of meta readings" ) if $combine;
        }
+       # We can only merge readings in a linear graph if:
+       # - they are contiguous with only one edge between them, OR
+       # - they are at equivalent ranks in the graph.
+       if( $self->linear ) {
+               my @delpred = $del_obj->predecessors;
+               my @keptsuc = $kept_obj->successors;
+               unless ( @delpred == 1 && $delpred[0] eq $kept_obj 
+                       && @keptsuc == 1 && $keptsuc[0] eq $del_obj ) {
+                       my( $is_ok, $msg ) = $self->relations->relationship_valid( 
+                               $kept_obj, $del_obj, 'collated' );
+                       unless( $is_ok ) {
+                               throw( "Readings $kept_obj and $del_obj can be neither concatenated nor collated" );
+                       } 
+               }
+       }
+       
        # We only need the IDs for adding paths to the graph, not the reading
        # objects themselves.
        my $kept = $kept_obj->id;