class Expr{
   protected:
      Expr();
};

class BinOp: public Expr{
   public:
/*
      char operation;
      Expr arg1, arg2;*/

      BinOp(char op, Expr a1, Expr a2);
};

class Const: public Expr{
   public:   
/*      int val; */
      Const(int v);
};

class Var: public Expr{
   public:
      Var(char* v);
};

/* Example: e=1+x
main()
{
   Expr e=BinOp('+',Const(1),Var("x")); 
}
*/