def append(list1, list2):
    result = []

    for element in list1:
        result.append(element)

    for element in list2:
        result.append(element)

    return result


def concat(lists):
    result = []

    for current_list in lists:
        for element in current_list:
            result.append(element)

    return result


def filter(function, seq):
    result = []

    for element in seq:
        if function(element):
            result.append(element)

    return result


def length(seq):
    count = 0

    for element in seq:
        count += 1

    return count


def map(function, seq):
    result = []

    for element in seq:
        result.append(function(element))

    return result


def foldl(function, seq, acc):
    for element in seq:
        acc = function(acc, element)

    return acc


def foldr(function, seq, acc):
    index = length(seq) - 1

    while index >= 0:
        acc = function(acc, seq[index])
        index -= 1

    return acc


def reverse(seq):
    result = []

    index = length(seq) - 1

    while index >= 0:
        result.append(seq[index])
        index -= 1

    return result