<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add(self, attribute):
 """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. """

        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