def forward(it, arg):
    # move iterator 'it' by calling 'next'(when 'arg' is None) or 'send'
    # return what 'next' or 'send' returns, or None if 'it' is at its end
    try:
        if arg==None:
            return it.next()
        else:
            return it.send(arg)
    except StopIteration: pass
    return Noneclass RECURSION(object):
    def __init__(self, *args):
        self.args=args
    
class TAIL(RECURSION):
    passdef heaprun(f, *args):
    stack=[f(*args)]
    param=None
    while(1):
        v=forward(stack[-1], param)
        if isinstance(v, RECURSION):
            if type(v)==TAIL:
                stack.pop()
            stack.append(f(*(v.args)))
            param=None
        else:
            stack.pop()
            if stack:
                param=v
            else:
                return v