#
# Utility functions.
#

import itertools

def unique(it):
    """Remove all duplicates from an iterable, leaving only the
    first instance of each duplicate.
    
    This is different from turning into a set because it preserves
    the order of the items."""
    
    it = iter(it)
    
    # This will raise StopIteration if the iterator is empty,
    # which is OK.
    head = it.next()
    
    # Now remove all items equal to the head.
    # This runs in quadratic time (and linear stack space), 
    # but never mind.
    yield head
    
    for i in itertools.ifilter(lambda i: i != head, unique(it)):
        yield i