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  Liskov substitution principle
       
           (LSP) The principle that
          object-oriented functions that use pointers or
          references to a base class must be able to use objects of
          a derived class without knowing it.
       
          Barbara Liskov first wrote it as follows: If for each object
          o1 of type S there is an object o2 of type T such that for all
          programs P defined in terms of T, the behaviour of P is
          unchanged when o1 is substituted for o2 then S is a subtype
          of T.
       
          A function that violates the LSP uses a reference to a base
          class and must know about all the derivatives of that base
          class.  Such a function violates the open/closed principle
          because it must be modified whenever a new derivative of the
          base class is created.
       
          [Liskov, B. Data Abstraction and Hierarchy, SIGPLAN
          Notices. 23(5), May 1988].
       
          (2001-09-14)