REBOL [
    Author: "Brett Handley"
    Title: "Series tools."
    Date: 18-Oct-2000
    Email: brett@codeconscious.com 
    Purpose: "Provide some additional function relating to series."
    Comment: "Yet another work in progress."
]

flatten: function [
    "Un-nests a series."
    series [series!]
    /deep "Works down through all levels of nesting."
    /in-place "Changes the passed series."
][cursor advance-num][
    either in-place [
        cursor: series
    ][
        cursor: copy/deep series
    ]
    while [not tail? cursor][
        either series? cursor/1 [
            either deep [advance-num: 1][advance-num: length? cursor/1]
            remove insert cursor cursor/1
            cursor: skip cursor advance-num
        ][
            cursor: next cursor
        ]
    ]
    head cursor
]