Why setting $contents when catching a rule?

[Akim Demaille]

I don't understand the following line in automake.in.  It seems very
wrong to me, but...

# Read Makefile.am and set up %contents.  Simultaneously copy lines
# from Makefile.am into $output_trailer or $output_vars as
# appropriate.  NOTE we put rules in the trailer section.  We want
# user rules to come after our generated stuff.
sub read_am_file
{
---------- CUT ----------
        elsif (/$RULE_PATTERN/o)
        {
            # Found a rule.
            $was_rule = 1;
            if (defined $contents{$1}
                && (@conditional_stack
                    ? ! defined $conditional{$1}
                    : defined $conditional{$1}))
            {
                &am_line_error ($1,
                                "$1 defined both conditionally and unconditionally");
            }
            # Value here doesn't matter; for targets we only note
            # existence.
=====> ???? $contents{$1} = 1;
            $targets{$1} = 1;
            my $cond_string = join ('', @conditional_stack);