anchor.diff:    | 'export' namedImports fromClause? ';'?
     | 'export' variableStatement
     | 'export' functionDeclaration
  +  | 'export' lexicalStatement
     | 'export' 'default' functionDeclaration
     | 'export' 'default' singleExpression ';'?
   ;
  
positive.code:  | debuggerStatement
   | importStatement
   | exportStatement
   ;
  
  importStatement
   : 'import' importClause fromClause ';'?
  
negative.code: ///     ThrowStatement
  ///     TryStatement
  ///     DebuggerStatement
  ///     TODO: remove import, export and lexical from statements
  statement
   : block
   | variableStatement
  