reserve x, y for set;
reserve D for non empty set;
reserve UN for Universe;
reserve C for Category;
reserve O for non empty Subset of the carrier of C;
reserve G,H for AddGroup;

theorem Th9:
  for F being GroupMorphism holds the Fun of F is additive
proof
  let F be GroupMorphism;
  the Fun of F = fun(F);
  hence thesis by Def11;
end;
