def append(list1: list, list2: list) -> list:
    return list1 + list2
def concat(lists: list[list]) -> list:  
    fin = []
    for l in lists:
        fin += l
    return fin
def filter(function, list: list) -> list: 
    fin = []
    for x in list:
        if(function(x)):
            fin.append(x)
    return fin
def length(list: list): 
    return len(list)
def map(function, list: list) -> list: 
    fin = []
    for x in list:
        fin.append(function(x))
    return fin
def foldl(function, list: list, initial: any) -> any:
    acc = initial
    for x in list:
        acc = function(acc, x)
    return acc
def foldr(function, list: list, initial: any) -> any: 
    acc = initial
    for x in list[::-1]:
        acc = function(acc, x)
    return acc
def reverse(list: list) -> list: 
    return list[::-1]