Adding new struct does not work correctly when there is more than 1 parent node

Summary:
Creating a new struct does not work correctly when there is more than 1 parent
node.

Symptoms:
The function $newstruct is used to explicitly create a new Struct with no
members. In case the referenced Struct that should be created has a parent that
is a collection of Struct nodes then this will not work as expected.

Consider the following code:

variables
struct vStruct
endvariables

vStruct->Library = $newstruct
vStruct->Library->Book = $newstruct
vStruct->Library->Book->Title = "Book Title 1"
vStruct->Library->Book->Author = "Author 1"
vStruct->Library->Book->Published = 2018

;;;vStruct->Library{-1} = $newstruct

vStruct->Library->Book

->Title = "Book Title 2"
vStruct->Library->Book

->Published = 2018

putmess vStruct->$dbgstringplain

When this code is executed then the output (in the Message Frame) will be:

[]
[Library]
[Book]
[Title] = "Book Title 1"
[Author] = "Author 1"
[Published] = 2018
[Book]
[Title] = "Book Title 2"
[Author] = "Author 2"
[Published] = 2018

In case the $newstruct instruction in the middle (that is commented out) is
enabled then the result is as follows:

[]
[Library]
[Book]
[Title] = "Book Title 1"
[] = ""
[Published] = 2018
[Library] = ""

Instead of creating a second Book Struct the second member of the Book Struct
is overwritten. As if the following line of code is executed:

vStruct->Library

->*

= $newstruct

It has to be said that the $newstruct instruction in question is ambiguous and
it would probably be better if Uniface would throw an error here. Maybe
something like error (USTRUCTERR_NOT_A_SINGLE_STRUCT; Tried to assign a struct
to multiple parents).