Define precondition and post-condition to a member function.

Define precondition and post-condition to a member function.
Precondition: A precondition is a condition that must be true on entry
to a member function. A class is used correctly if preconditions are
never false. An operation is not responsible for doing anything
sensible if its precondition fails to hold. For example, the interface
invariants of stack class say nothing about pushing yet another element
on a stack that is already full. We say that isful() is a precondition
of the push operation. Post-condition: A post-condition is a condition
that must be true on exit from a member function if the precondition
was valid on entry to that function. A class is implemented correctly
if post-conditions are never false. For example, after pushing an
element on the stack, we know that isempty() must necessarily hold.
This is a post-condition of the push operation.

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