FORSKIP port from Rebol2 to Red.

forskip.red

Red [
  Title: "FORSKIP function"
  Author: "Nenad Rakocevic"
  Purpose: "Direct port of the REBOL2 forskip function"
]

forskip: func [
    "Evaluates a block for periodic values in a series."
    'word [word!] "Word set to each position in series and changed as a result"
    skip-num [integer!] "Number of values to skip each time"
    body [block!] "Block to evaluate each time"
    /local orig result
][
    unless positive? skip-num [cause-error 'script 'invalid-arg reduce [skip-num]]
    unless any [
        series? get word
        port? get word
    ][cause-error 'user 'message ["forskip/forall expected word argument to refer to a series or port!"]]
    orig: get word
    while [any [not tail? get word (set word orig false)]] [
        set/any 'result do body
        set word skip get word skip-num
        get/any 'result
    ]
]