Cannot parse formulas with union operators near other operators

[pillowfication]

The parser can only seem to handle unions when it's the only operator in its group.

• =A:A,B:B,C:C works
• =(A:A,B:B)+C:C works

However, it fails if it sees another operator in the same level.

• =A:A,B:B+C:C fails
• =A:A,B:B,@C:C fails

Is it possible to have the union operator treated like a binary operator instead of a function call, deviating from the paper this project's grammar is based on to more closely match how Excel interprets it?