'Add an association type and session type to the allowed
types list. The assocation/session pairs are tried in the
order that they are added.'

def addAllowedType(self, assoc_type, session_type=None):

    if (self.allowed_types is None):
        self.allowed_types = []
    if (session_type is None):
        available = getSessionTypes(assoc_type)
        if (not available):
            raise ValueError(('No    session    available    for    association    type    %r' % (assoc_type,)))
        for session_type in getSessionTypes(assoc_type):
            self.addAllowedType(assoc_type, session_type)
    else:
        checkSessionType(assoc_type, session_type)
        self.allowed_types.append((assoc_type, session_type))
