anchor.diff:     : '\'' ('\'\'' | ~ ('\''))* '\''
      ;
   
  +
   NUM_INT
  -   : ('0' .. '9') + 
  -   ;
  +   : ('0' .. '9') +
  +   ;
  +
   
   NUM_REAL
      : ('0' .. '9') + (('.' ('0' .. '9') + (EXPONENT)?)? | EXPONENT)
  
positive.code:    ;
  
  
  WS
     : [ \t\r\n] -> skip
     ;
  
negative.code:    | unsignedConstant
     | set
     | NOT factor
     | bool
     ;
  
  unsignedConstant
  