blah.coffee

  ForBody: [
    o 'FOR Range ForExtra',                     -> extend {source: LOC(2) new Value($2)}, $3
    o 'ForStart ForSource ForExtra',            ->
      $2.own = $1.own; $2.name = $1[0]; $2.index = $1[1]
      extend $2, $3
  ]

  ForStart: [
    o 'FOR ForVariables',                       -> $2
    o 'FOR OWN ForVariables',                   -> $3.own = yes; $3
  ]

  # An array of all accepted values for a variable inside the loop.
  # This enables support for pattern matching.
  ForValue: [
    o 'Identifier'
    o 'ThisProperty'
    o 'Array',                                  -> new Value $1
    o 'Object',                                 -> new Value $1
  ]

  # An array or range comprehension has variables for the current element
  # and (optional) reference to the current index. Or, *key, value*, in the case
  # of object comprehensions.
  ForVariables: [
    o 'ForValue',                               -> [$1]
    o 'ForValue , ForValue',                    -> [$1, $3]
  ]

  # The source of a comprehension is an array or object with an optional guard
  # clause. If it's an array comprehension, you can also choose to step through
  # in fixed-size increments.
  ForSource: [
    o 'FORIN Expression',                       -> source: $2
    o 'FOROF Expression',                       -> source: $2, object: yes
  ]

  ForExtra: [
    o '',                                       -> {}
    o 'BY Expression',                          -> step:  $2
    o 'WHEN Expression',                        -> guard: $2
    o 'BY Expression WHEN EXPRESSION',          -> step:  $2, guard: $4
    o 'WHEN Expression BY EXPRESSION',          -> guard: $2, step:  $4
  ]
```