A Constant is a special kind of Expression.
A Constant has a integer value.
When created the value is given to the constant to be stored.
When evaluated a Constant returns its value
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A Operation is a special kind of Expression.
A Operation has a left and right hand Expression plus an operator.
When created these objects are given to the Operation.
When evaluated an Operation evaluates its left and right expressions and
returns the result of applying the operator to their values.
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An Addition is a special kind of Operator.
When applied, an Addition is given two integers and it returns their sum.
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An Multiplication is a special kind of Operator.
When applied, an Multiplication is given two integers and it returns their product.
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