if ( $rel eq 'transposition' || $rel eq 'repetition' ) {
# Check that the two readings do (for a repetition) or do not (for
# a transposition) appear in the same witness.
+ # TODO this might be called before witness paths are set...
my %seen_wits;
map { $seen_wits{$_} = 1 } $c->reading_witnesses( $source );
foreach my $w ( $c->reading_witnesses( $target ) ) {