operator: expand-hierarchy

    description:
        expand-hierarchy ket
        expand the category hierarchy
        
    examples:
        expand-hierarchy |a: b: c: d>
            |a> + |a: b> + |a: b: c> + |a: b: c: d>
        
        expand-hierarchy |a: b: c: d: e: f>
            |a> + |a: b> + |a: b: c> + |a: b: c: d> + |a: b: c: d: e> + |a: b: c: d: e: f>
        
        expand-hierarchy |a: b: c: x: y>
            |a> + |a: b> + |a: b: c> + |a: b: c: x> + |a: b: c: x: y>
        
        intersection(expand-hierarchy |a: b: c: d: e: f>, expand-hierarchy |a: b: c: x: y>)
            |a> + |a: b> + |a: b: c>
        
        expand-hierarchy |animal: mammal: dog>
            |animal> + |animal: mammal> + |animal: mammal: dog>
       
    see also:
        extract-category, extract-value, category-depth, inherit