description:
find-leading-category |x>
find the leading category
examples:
-- if there is no category, then return unchanged:
find-leading-category |fish>
|fish>
find-leading-category |animal: mammal: dog>
|animal>
find-leading-category |a: b: c: d: e: f>
|a>
see also:
remove-leading-category, extract-category, extract-value