/**
   abstract class for BooleanExpression

   A BooleanExpression, in this context, is the parse tree of an 
   expression involving and, or, not, and implies such 
   ( ( a ^ b ) v ( c > (! d ) ) )


   At each leaf of the tree, there is either a variable (which is a single
   lowercase letter) or there is a literal value of T (true) or F (false).

   BooleanVariable and BooleanLiteral are both subclasses of
   BooleanExpression that represent these cases.

   Interior nodes of the three are either of class BinaryBooleanExpression, or
   UnaryBooleanExpression.

   UnaryBooleanExpression nodes have only one child.

   BinaryBooleanExpression nodes have exactly two children.

 */

public abstract class BooleanExpression {
    public boolean getValue();
    public String toString();
}