anchor.diff:  LINE_COMMENT
       : '#' ~('\n'|'\r')* '\r'? '\n' {$channel=HIDDEN;}
       ;
  -
  
positive.code:         return true;
      }
      ;
      
  arithmatic_opcode returns [I0Instruction::I0Opcode retval]
      : 'add'
      {
  
negative.code:         $retval = $op4.retval;
      }
      ;
  
  immediate_operand returns [I0Instruction::I0Operand retval]
      : '$' integer_value
      {
          $retval = I0Instruction::I0Operand(I0Instruction::Immediate, $integer_value.retval, I0Instruction::ConvSigned64);
      }
      | '$' fp_value
  