anchor.diff:     | expression OPERATOR expression
      | LPAREN expression RPAREN
      | expression '?' expression ':' expression
  +   | map
      ;
   
   RESOURCEREFERENCE
  -   : [a-zA-Z] [a-zA-Z0-9_-]* RESOURCEINDEX? '.' ([a-zA-Z0-9_.-] RESOURCEINDEX?)+ 
  +   : [a-zA-Z] [a-zA-Z0-9_-]* RESOURCEINDEX? '.' ([a-zA-Z0-9_.-] RESOURCEINDEX?)+
      ;
   
   RESOURCEINDEX
  
positive.code:    | '||'
     ;
  
  LPAREN
     : '('
     ;
  
negative.code:    ;
  
  map
     : '{' argument* (',' argument)* '}'
     ;
  
  string
  