anchor.diff:     | MINUS
      ;
   
  +bool
  +   : TRUE
  +   | FALSE
  +   ;
  +
   string
      : STRING_LITERAL
      ;
  
positive.code:    | unsignedConstant
     | set
     | NOT factor
     ;
  
  unsignedConstant
  
negative.code:    : '\'' ('\'\'' | ~ ('\''))* '\''
     ;
  
  
  NUM_INT
     : ('0' .. '9') +
     ;
  
  
  NUM_REAL
     : ('0' .. '9') + (('.' ('0' .. '9') + (EXPONENT)?)? | EXPONENT)
  