resolve_one_grouping: Resolve a single by entry to a grouping triple
Description
Internal dispatch helper used by resolve_groupings(). Identifies
whether entry is a stem string or an explicit c(part, cnt) vector
and delegates to stem_grouping() or explicit_grouping() accordingly.
Raises an error if entry matches neither shape.
Usage
resolve_one_grouping(entry)
Value
a list(name, part, cnt) triple
Arguments
- entry
one element of the user-facing by argument