code: def add_instruction (self, instr):
          assert(isinstance(instr, Instruction))
          self.instruction_list.append(instr)
          if instr.lhs not in self.defined_variables:
              if isinstance(instr.lhs, Variable):
                  self.defined_variables.append(instr.lhs)
          if isinstance(instr, EqInstruction):
              if isinstance(instr.rhs, Variable):
                  if instr.rhs not in self.used_variables:
                      self.used_variables.append(instr.rhs)
          else:
              if isinstance(instr.rhs_1, Variable):
                  if instr.rhs_1 not in self.used_variables:
                      self.used_variables.append(instr.rhs_1)
              if isinstance(instr.rhs_2, Variable):
                  if instr.rhs_2 not in self.used_variables:
                      self.used_variables.append(instr.rhs_2)