function mergesort(m)
    var list left, right, result
    if length(m) ? 1
        return m
    else
        var middle = length(m) / 2
        for each x in m up to middle
            add x to left
        for each x in m after middle
            add x to right
        left = mergesort(left)
        right = mergesort(right)
        result = merge(left, right)
        return result

function merge(left,right)
    var list result
    while length(left) > 0 and length(right) > 0
        if first(left) ? first(right)
            append first(left) to result
            left = rest(left)
        else
            append first(right) to result
            right = rest(right)
    if length(left) > 0 
        append rest(left) to result
    if length(right) > 0 
        append rest(right) to result
    return result