code: func isNullSafeAccess(n ast.Node) bool {
  	switch node := n.(type) {
  	case *ast.DataRefIndexNode:
  		return node.NullSafe
  	case *ast.DataRefKeyNode:
  		return node.NullSafe
  	case *ast.DataRefExprNode:
  		return node.NullSafe
  	}
  	panic("unexpected")
  }
docs: // isNullSafeAccess returns true if the data ref access node is a nullsafe
  // access.