sub compatible {
	my( $l, $r ) = @_;

	# underscores are insignificant
	tr/_/\0/ for $l, $r;

	# cancel out identical values
	my $xor = $l ^ $r;

	# convert to bitmasks
	tr/\0/\377/c for $l, $r;

	my $mask = $l & $r;

	# masked chars must be identical
	return !1 if ( $xor & $mask ) =~ tr/\0//c;

	# and there may not be dupes of non-identical characters
	return 0 == grep {
		my $char = substr( $xor, $_, 1 );
		$char ne "\0" and index( $xor, $char, $_ + 1 ) != -1
	} 0 .. length( $xor ) - 1;
}