Higher-Order Parsing

New tools

Or "do what A does, but only if the result satisfies some condition":

        sub side_condition {
          my ($A, $condition) = @_;
          my $parser = sub {    
            my $in = shift;
            my ($res, $out) = $A->($in)
                or return;
            unless ($condition->($res)) { return; }
            return ($res, $out);
          };
        }