Evaluation of expression within parenthesis

Good day everyone.

I have to write several records into a file (or table), for which there is
al field (or column), that is a sequential number, but for some key field
combination.

Say that I have some USER, TASK, SEQUENCE key combination, for which the
SEQUENCE would start from 1 through an arbitrary ending number.

USER1 ADD 1
USER1 ADD 2
USER1 ADD 3
and so on.

I was trying to do this for the sequence:

USER = USER1;
TASK = 'ADD';
SEQN = (start_seq_n += 1);

where start_seq_n would be initially 1. I was expecting, like in "C", that
the compiler would first evaluate the internal increment and assignment and
just then assign to SEQN the result, so as to save some lines of code. I
see it gives me an error. I thought this was possible in RPGLE.

If not that I'm doing something wrong, and the error is real, then I'd
suggest that the compiler be able to evaluate that as a recursive RVALUE
and then an LVALUE.

JS