def length(list):
    count = 0
    for item in list:
        count +=1
    return count

        
def append(list1, list2):
    result = []
    for item in list1:
        result.append(item)
    for item in list2:
        result.append(item)
    return result


def concat(lists):
    result = []
    for box in lists:
        for item in box:
            result.append(item)
    return result


def filter(function, list):
    result = []
    for item in list:
        if function(item):
            result.append(item)
    return result


def map(function,list):
    result = []
    for item in list:
        result.append(function(item))
    return result


def reverse(list):
    result = []
    for item in list:
        result = [item] + result
    return result


def foldl(function,list,initial):
    accamulator = initial
    for item in list:
        accamulator = function(accamulator, item)
    return accamulator


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