*      UPDATE tab SET col.fld1.subfld1 = x, col.fld2.subfld2 = y
      * The two expressions produced by the parser will look like
      *      FieldStore(col, fld1, FieldStore(placeholder, subfld1, x))
 -    *      FieldStore(col, fld2, FieldStore(placeholder, subfld2, x))
 +    *      FieldStore(col, fld2, FieldStore(placeholder, subfld2, y))
      * However, we can ignore the substructure and just consider the top
      * FieldStore or ArrayRef from each assignment, because it works to
      * combine these as
      *      FieldStore(FieldStore(col, fld1,
      *                            FieldStore(placeholder, subfld1, x)),
 -    *                 fld2, FieldStore(placeholder, subfld2, x))
 +    *                 fld2, FieldStore(placeholder, subfld2, y))
      * Note the leftmost expression goes on the inside so that the
      * assignments appear to occur left-to-right.
      *