def append(list1, list2):
    result = []
    for x in list1:
        result = result + [x]
    for x in list2:
        result = result + [x]

def concat(lists):
    result = []
    for list in lists:
        append(result, list)
    return result

def filter(function, list):
    result = []
    for x in list:
        if function(x):
            result = result + [x]
    return result

def length(list):
    lng = 0
    for i in list:
        lng += 1
    return lng

def map(function, list):
    result = []
    for x in list:
        if function(x) not in result:
            result = result + [x]
    return result

def foldl(function, list, initial):
    acc = initial
    for x in list:
        acc = function(acc, x)
    return acc

def foldr(function, list, initial):
    return foldl(function, reverse(list), initial)

def reverse(list):
    result = list[::-1]
    return result