code: func (a *AggregateFuncExtractor) Enter(n ast.Node) (ast.Node, bool) {
  	switch n.(type) {
  	case *ast.AggregateFuncExpr:
  		a.inAggregateFuncExpr = true
  	case *ast.SelectStmt, *ast.UnionStmt:
  		return n, true
  	}
  	return n, false
  }
docs: // Enter implements Visitor interface.