operator: extract-category

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

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