extract-headless ket
categories are separated by ": "
with the most general category on the left, and the most specific on the right
for example |animal: mammal: dog>
has the most general category "animal", then the category "mammal" and then finally the "value" dog
This list of categories and the value can be considered a type of list
extract-headless removes the head of the list, if it has one.
If the ket has no category, then return the ket unchanged.