A UNION B JOIN C wrong output

[s-minoo]

Issue

When using join expressions, ShExML engine produces inconsistent results based on the ordering of the expressions used in A UNION B JOIN C.

I interpreted it as follows:

• If B and C are equal, get the corresponding "row" from A provided that particular "row" is present in either B or C.

If yes, there are currently two issues with it:

1. Switching B and C will produce an error if "C" is not from the same "source + iterator" as A even though the execution syntax should not change.
2. Results from A is ignored if B and C are never equal, and all the results from B is produced.

A simple test case using a variation of the input from mappingChallenges/JoinChallenge.scala can reproduce the aforementioned issues.

I have attached the input shexml files and screenshots of the respective problems:

1) file: errored_a_c_neq.txt screenshot:

2) file: A_ignored.txt screenshot: