extract-value ket
extract the value, ie, remove the category, from the given 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
categories can be considered a type for the object
and values are specific examples of that type
Eg, |food: apple> says apple is a type of food, and the value of |food: apple> is apple