Love this solution! Using upto and downto in the context of a lift is just delightful.

A small improvement (arguably) could be to change stop to a proper bool throughout and then change

stop = 0
lift.reject!{|n| n == floor && (stop = 1)}

to

stop = lift.delete(floor)

You could decompose the signature in the method definition if you like: def tribonacci({a,b,c}, n) do