StringRiffle handling multiple nesting levels not implemented

[den-run-ai]

https://reference.wolfram.com/language/ref/StringRiffle.html

[sn6uv]

We could implement this using Riffle and StringJoin.

For example: (there are many cases this is just the simplest)

In[1]:= StringRiffle[x_List] := StringJoin[Riffle[x, " "]]
In[2]:= StringRiffle[{"a", "b", "c", "d", "e"}]
Out[2]= a b c d e

[rocky]

As of 1.1.0rc1 and 44ee56b34 (by Long Nguyen), StringRiffle is in strings.py. Handling the different kinds of levels though is not.

Note however that the Rubi package doesn't need this form, but only one of the forms handled as of 1.1.0rc1