built in operator: how-many

    description:
        how-many sp
        returns the number of kets in a superposition

    examples:
        how-many |>
            |number: 0>
                    
        how-many split |a b c d e f g>
            |number: 7>
        
        -- learn some knowledge:
        friends |Fred> => |Jack> + |Harry> + |Ed> + |Mary> + |Rob> + |Patrick> + |Emma> + |Charlie>
        friends |Sam> => |Charlie> + |George> + |Emma> + |Jack> + |Rober> + |Frank> + |Julie>

        -- ask how many friends:
        how-many friends |Fred>
            |number: 8>
        
        how-many friends |Sam>
            |number: 7>

    see also:
        show-many, measure-currency