input: expr eof { $item[1] }eof: /^\Z/expr: <leftop: term /([-+])/ term>        { eval join(' ', @{$item[1]}); }term: <leftop: factor /([*\/])/ factor>        { eval join(' ', @{$item[1]}); }factor: neg(?) number            { my $neg = $item[1]->[0] || ''; $neg . $item[2] }      | '(' expr ')'  { $item[2] }neg: '-'number: /[1-9]\d*/