import sys
# requires Python 2.5
assert sys.version_info[1] >= 5, "Requires Python Coroutines. See PEP 0342"

def pushbackiter(iterator):
    """converts 'iterator' into a coroutine, with limited pushback

    >>> it = pushbackiter([1,2,3,4,5])
    >>> it.next()
    1
    >>> it.send(1)
    None
    >>> it.next()
    1
    """
    for elem in iterator:
        pushback = yield elem         # yield 'elem' to the caller of '.next()', 'pushback' will become == 'None'
        if pushback is not None:      # this implementation is unable to take back a None value
            yield None                # yield 'None' to the caller of '.send(x)'
            pushback = yield pushback # sets 'pushback' to the value passed in via '.send(x)'

def intervals(xs):
    """groups 'xs' into a pair generator indicating consecutive ranges

    >>> list(intervals([1,2,3,5,7,8,12,15,16])
    [(1,3),(5,),(7,8),(12,),(15,16)]
    """
    def peek(xs):
        """helper function does the pushback for us"""
        x = xs.next()
        xs.send(x)
        return x

    xs = pushbackiter(xs)
    for x in xs:
        x_ = x
        while True:                    # locate the last successor of x
            try:
                y = peek(xs)
                if x_ + 1 == y:
                    x_ = y
                    y = xs.next()
                    continue
                elif x_ == x:          # x_ + 1 is not a successor and we have not iterated off the left bound
                  x_ = None
            except StopIteration:
                x_ = None

            break

        if x_ is not None:
            yield (x, x_)
        else:
            yield (x,)