9 my ($src, $dst) = /(\w) must be.* (\w) can/;
10 $prereqs{$dst}->{$src} = 1;
17 my @avail = sort grep { keys %{ $prereqs{$_} } == 0 } keys %steps;
18 my $finished = $avail[0];
20 delete $steps{$finished};
21 for my $p (keys %prereqs) {
22 delete $prereqs{$p}->{$finished};