anchor.diff:  
   defaultExpression
       : rangeIntervalEntryExpression 
  -      ('?'^ defaultExpression)?
  +      (QMARK^ defaultExpression)?
       ;
   
   //I wonder if it would it be cleaner to give 
  
positive.code:     :   '~'
      ;
  
  QMARK
      :   '?'
      ;
  
  COLON
      :   ':'
      ;
  
negative.code:     ;
      
  primary
      : base selector*
      -> ^(EXPR base selector*)
      ;
  
  postfixOperator
  