'A valid state is one of:
- None
- installed
- upgraded
- head
- linked
- unlinked
- absent'

@classmethod
def valid_state(cls, state):

    if (state is None):
        return True
    else:
        return (isinstance(state, string_types) and (state.lower() in ('installed', 'upgraded', 'head', 'linked', 'unlinked', 'absent')))
