myassembler（大学作业）

				public class Add extends Expression {				   private Expression firstOp, secondOp;				   				   public Add(Expression f, Expression s) {				      firstOp = (Expression)f.clone();				      secondOp = (Expression)s.clone();				   }				   				   public int eval() {				      return firstOp.eval()+secondOp.eval();				   }				   				   public Object clone() {				      return new Add(firstOp, secondOp);				   }				   				   public String toString() {				      return firstOp.toString()+secondOp.toString()+labelling()+"ADD\n";				   }				}