Helper method for defining parse actions that require matching at
    a specific column in the input text.

def matchOnlyAtCol(n):
    """Helper method for defining parse actions that require matching at
    a specific column in the input text.
    """
    def verifyCol(strg,locn,toks):
        if col(locn,strg) != n:
            raise ParseException(strg,locn,"matched token not at column %d" % n)
    return verifyCol