# Determines if two hashes have different keys
# If the hashes are different:
#    In list context returns two array refs, each 
#        containing the missing elements from the other
#    In scalar context returns an AOA, with a similar 
#        content as above
# Otherwise
#    Returns the empty list or undef as appropriate.
# Thus
#    A true return means they hashes are different, 
#        a false means they have identical keys

sub hash_key_diff {
   my ($x,$y)=@_;
   my (@x,@y);
   foreach my $k (keys %$x) {
     push @y,$k unless exists $y->{$k};
   } 
   foreach my $k (keys %$y) {
     push @x,$k unless exists $x->{$k};
   }
   return @x||@y ? wantarray ? (\@x,\@y) : [\@x,\@y]
                 : ()
}