'Add an attribute to this attribute exchange request.
@param attribute: The attribute that is being requested
@type attribute: C{L{AttrInfo}}
@returns: None
@raise KeyError: when the requested attribute is already
present in this fetch request.'

def add(self, attribute):

    if (attribute.type_uri in self.requested_attributes):
        raise KeyError(('The    attribute    %r    has    already    been    requested' % (attribute.type_uri,)))
    self.requested_attributes[attribute.type_uri] = attribute
