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