REL — 9/Relationship Service specification

CosGraphs::Node interface provides a function add_role() which helps in adding a new role to the related
object the Node is representing. The specifications suggests to raise an exception DuplicateRoleType
if the new role is same, subtype, supertype of one of the role of the Node object.

What if the user of function calls add_role() with a role for an object which is not represented by this node.
Definitely we cannot add the new role to the Node. Only thing we can do here is return out of function.
But user will not be aware that the function has failed, since the return type is void.