def append(list1, list2):
    result = []
    for item in list1 :
        result = result + [item]
    for item in list2 :
        result = result + [item]
    return result
def concat(lists):
    result = []
    for item in lists:
        result = append(result, item)
    return result
def filter(function, list):

    res = []
    for item in list:
        if function(item) :
            res = res + [item]
    return res
def length(list) :
    counter = 0
    for item in list:
        counter = counter + 1
    return counter
def map(function, list) :
    res = []
    for item in list:
        res = res + [function(item)]
    return res
def foldl(function, list, initial):
    final = initial
    for item in list:
        final = function(final, item)
    return final
def foldr(function, list, initial):
    final = initial
    for item in reverse(list):
        final = function(final, item)
    return final
def reverse(list):
    result = []
    for item in list:
        result = [item] + result
    return result