anchor.diff:      : [abcxyzijABCXYZIJ]
       ;
   
  -unary_opcode
  +unaryOpcode
       : JSR
       | INT
       | IAG
  
positive.code: STI : 'STI' | 'sti' ;
  STD : 'STD' | 'std' ;
  
  argument_a
      : REGISTER
      | location_in_register
      | location_offset_by_register
      | POP
      | PEEK
      | pick
  
negative.code: grammar Dcpu;
  
  program
      : (label | instruction | dataSection)+ EOF
      ;
  
  label
  