=head1 NAME

Class::AutoDB::Collection - Compare two collection definitions and
maintain differences

=head1 SYNOPSIS

This is a helper class for Class::AutoDB::Registry to to process
differences between in-memory and saved registries

 use Class::AutoDB::CollectionDiff;
 use Class::AutoDB::Collection;
 my $diff=new Class::AutoDB::CollectionDiff
   (-baseline=>$saved, -change=>$in_memory);
 if ($diff->is_sub) {               # is new collection subset of saved one?
   $registry=$saved_registry;       # if so, use saved one
 } elsif  ($diff->is_different) {   # else get changes
   my %new_keys=$diff->new_keys;    # 
   my @expanded_collections=$diff->expanded_collections;
 } else {
   die “New collection inconsistent with old”;
 }

=head1 DESCRIPTION

This class compares two collection definitions and records their
differences. The first collection is considered the baseline, and
differences are reported relative to it. This class I<does not talk to
the database>.

=cut