SelectWithUnionQuery (children 1)
 ExpressionList (children 1)
  SelectQuery (children 3)
   ExpressionList (children 2)
    Function materialize (children 1)
     ExpressionList (children 1)
      Literal \'a\'
    Literal \'a\' (alias key)
   ExpressionList (children 1)
    Identifier key
   Set
