Ngrams with coroutines 2

gistfile1.py

@coroutine
def filter_chars(accepted_chars,target):
    """ A coroutine to filter out unaccepted chars. 
    Accepts one char at a time  """
    while True:
        c = (yield)
        if c.lower() in accepted_chars:
            target.send(c.lower())

@coroutine
def counter(matrix):
    """ A counter sink """
    while True:
        a, b = (yield)
        matrix[pos[a]][pos[b]] += 1