'Returns the count of the given keyword in the header, similar to
`list.count` if the Header object is treated as a list of keywords.
Parameters
keyword : str
The keyword to count instances of in the header'

def count(self, keyword):

    keyword = Card.normalize_keyword(keyword)
    if (keyword not in self._keyword_indices):
        raise KeyError('Keyword    {!r}    not    found.'.format(keyword))
    return len(self._keyword_indices[keyword])
