- my @oldpieces = ('', split / /, $1);
- my $parentix = -1 + scalar @{ $old_upstream->{Parents} };
- foreach my $i (0..$#oldpieces) {
- my $n = $oldpieces[$i];
- $piece->($n, Old => $old_upstream->{CommitId}.'^'.$parentix);
+ my @oldpieces = (split / /, $1);
+ my $old_n_parents = scalar @{ $old_upstream->{Parents} };
+ if (@oldpieces != $old_n_parents) {
+ snag 'upstream-confusing', sprintf
+ "previous upstream combine %s".
+ " mentions %d pieces (each implying one orig commit)".
+ " but has %d parents",
+ $old_upstream->{CommitId},
+ (scalar @oldpieces),
+ $old_n_parents;
+ } elsif ($oldpieces[0] ne '.') {
+ snag 'upstream-confusing', sprintf
+ "previous upstream combine %s".
+ " first piece is not \`.'",
+ $oldpieces[0];
+ } else {
+ $oldpieces[0] = '';
+ foreach my $i (0..$#oldpieces) {
+ my $n = $oldpieces[$i];
+ $piece->($n, Old => $old_upstream->{CommitId}.'^'.($i+1));
+ }