anchor.diff:     ;
   
   identifierchain
  -   : (IDENTIFIER | IN) index? (DOT identifierchain)*
  +   : (IDENTIFIER | IN | VARIABLE) index? (DOT identifierchain)*
      | STAR (DOT identifierchain)*
      ;
   
  
positive.code:    | MULTILINESTRING
     ;
  
  identifier
     : IDENTIFIER
     | 'variable'
     ;
  
  fragment DIGIT
     : [0-9]
     ;
  
negative.code:    : '+' NUMBER
     | '-' NUMBER
     | NUMBER
     ;
  
  VARIABLE
     : 'variable'
     ;
  
  IN
  