Revisit problem 3.3 (exercise about the Composite pattern).
Extend the implementation of the "while" language with
expressions, which were left empty.  The abstract syntax of
expressions is

  Expr := Variable | Operation Expr Expr
  
where "Variable" is an identifier or a literal and "Operation" is
a familiar infix operation, such as add, sub, mul, etc.

Design, code and test three programs:

1. The objects representing variables (and literals) are not shared.
2. The objects representing variables (and literals) contain only
   intrinsic state and are shared by the statements.
3. The objects representing variables (and literals) contain both
   intrinsic and extrinsic state. They are not shared by the
   statements, but share themselves the objects with intrinsic
   state only. For this exercise's purpose, the extrinsic state
   is the line in a source program with a variable occurs.

As in the original problem, it suffices to contruct the
representation of a program and print it.

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An important issue in the Flyweight pattern concerns the state of
objects.  We distinguish between intrinsic and extrinsic states.
Intrinsic is stored in a shared object and obviously does not
depend on the context of an object.  Extrinsic depends on the
context of an object and cannot be shared.

[For example, in the expressions of the "while" language, the
value of a variable is intrinsic, but the line (and column) in a
source program where the variable occurs is extrinsic.]