Every definition parser repeated the same triplet per optional
attribute — `if slot.is_some() { return duplicate_clause_err(...) }
slot = Some(parse_X()?)`. Factor the check-then-assign into one
helper, set_once(slot, production, subject, clause, value), invoked as
`set_once(&mut doc, prod, subj, "doc", self.parse_doc()?)?`. The match
on peek_head_ident() and the per-clause production/subject/clause
arguments are unchanged, so every duplicate-clause error string stays
byte-identical (the *_rejects_duplicate_*_clause tests pin them).
The helper checks is_some() before the parser runs, so a duplicate
clause still reports at the offending clause's position. Arms that do
more than a plain check+assign (the bool flags drop-iterative/kernel,
the param-in nested map, the suppress/method Vec pushes) are left
inline.
Brummel
5cd2a964e3