def zipWith(f,ls,xs):
    for i in range(len(ls)):
        yield f(ls[i],xs[i])

def all(f, ls):
    for x in ls:
        if not f(x): return False
    return True

def splitWhen(f, ls):
    b, rs, xs = False, [], []
    for x in ls:
        if f(x): b = True
        if b: xs += [x]
        else: rs += [x]
    return rs, xs

def tail(ls):
    return list(ls)[1:]

def take(n,ls):
    for x in ls:
        if n < 1: break
        yield x
        n -= 1

def drop(n,ls):
    for x in ls:
        if n < 1: yield x
        n -= 1

def splitAt(i, ls):
    ls = list(ls)
    return ls[:i], ls[i:]

def partBy(n, ls):
    rs = []
    while len(ls) >= n:
        xs, ls = splitAt(n,ls)
        rs.append(list(xs))
    return rs

def concat(ls):
    for xs in ls:
        for x in xs:
            yield x


def dropWhile(f, ls):
    b = False
    for x in ls:
        if not b:
            if f(x): continue
            else:
                b = True
        if b:
            yield x

def unfold(f, x):
    """
    def f(x):
        if x > 0: return x, x-1

    print(list(unfold(f, 5)))
    """
    while True:
        r = f(x)
        if not r: break
        v, x = r
        yield v


def nub(ls):
    rs = []
    for x in ls:
        if x not in rs:
            rs.append(x)
    return rs


def cartProd(ls, xs):
    for i in ls:
        for x in xs:
            if not i is x:
                yield i, x


def numEqPrefix(ls, xs):
    acc = 0
    for i in range(len(ls)):
        try:
            if ls[i] == xs[i]:
                acc += 1
        except:
            break
    return acc


def pairs2list(ls):
    rs = []
    for a, b in ls:
        if not a in rs:
            rs.append(a)
        if not b in rs:
            rs.append(b)
    return rs