----- LAW OF DEMETER (CLOS, object version) -------------
All function calls inside a method M must have
one of the following method selection argument objects:
  - M's argument objects or
  - slot values of a method selection argument class of M.
(Objects created by the method and objects in global 
variables are viewed as being passed by arguments.
A method selection argument is an argument which is used
for identifying the applicable methods.)
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