composer calculator
  (<WS>?) <addition|multiplication|term> (<WS>?)
  rule addition: [<addition|multiplication|term> (<WS>?) <'[+-]'> (<WS>?) <multiplication|term>] ->
    \(when <?($(2) <='+'>)> do $(1) + $(3) !
      otherwise $(1) - $(3) !
    \)
  rule multiplication: [<multiplication|term> (<WS>?) <'[*/]'> (<WS>?) <term>] ->
    \(when <?($(2) <='*'>)> do $(1) * $(3) !
      otherwise $(1) ~/ $(3) !
    \)
  rule term: <INT|parentheses>
  rule parentheses: (<'\('> <WS>?) <addition|multiplication|term> (<WS>? <'\)'>)
end calculator

'(100 - 5 * (2+3*4) + 2) / 2' -> calculator -> !OUT::write
'
' -> !OUT::write