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extract-head 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-head returns the first element, or head, of this list

-- if there is no category, then return unchanged:
extract-head |dog>
    |dog>

extract-head |animal: mammal: dog>
    |animal>

-- an abstract example:
extract-head |a: b: c: d: e: f>
    |a>