// This class is the superclass of the classes for machine instructions
public abstract class Instruction {
    
    // Constructor: an instruction with label l and opcode op
    // (op must be an operation of the language)
    public Instruction(String l, String op) {
    }
    
    // = the representation "label: opcode" of this Instruction
    public String toString() {
        return "";
    }
    
    // Execute this instruction on machine m. 
    public abstract void execute(Machine m);
}