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

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

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

def length(l):
    result = 0
    for item in l:
        result +=1
    return result