anchor.diff:      : 'func' function
       ;
   
  -selector
  -    : '.' IDENTIFIER
  -    ;
  -
   index
       : '[' expression ']'
       ;
  
positive.code:     ;
  
  methodExpr
      : receiverType '.' IDENTIFIER
      ;
  
  receiverType
      : typeName
      | '(' ('*' typeName | receiverType) ')'
      ;
  
  eos
  
negative.code:     : string
      ;
  
  /*topLevelDecl
      : declaration
      | functionDecl
      | methodDecl
      ;
  */
  
  declaration
      : constDecl
  