$v = ${$pairs{$ca}}[0];
            if ($types{$ca}) {
                $v = 0 unless defined $v;
                $op = $NUM_OPS{$co};
            } else {
                $v = '' unless defined $v;
                $op = $STR_OPS{$co};
            }
            die("Undefined operation $co.\n") unless $op;
            $r &= $op->($v, $cv);