REBOL [
    Title: "System tools"
    Author: "Brett Handley"
    Date: 20-May-2001
    Acknowledgements: {Larry Palmiter}
]

value-words?: function ["Returns a list of words with values."] [result word-list word][
    result: make block! divide length? word-list: first system/words 2
    for i 1 length? word-list 1 [
        word: word-list/:i
        if not error? try [get in system/words :word][
            insert tail result word
        ]
    ]
    RETURN result
]

new-value-words?: function [
    "Returns words set by the expression."
    expression
][before after][
    before: value-words?
    do expression
    after: value-words?
    RETURN exclude after before
]