procedure comb(integer pool, needed, done=0, sequence chosen={})
    if needed=0 then    -- got a full set
        ?chosen         -- (or use a routine_id, result arg, or whatever)
        return
    end if
    if done+needed>pool then return end if -- cannot fulfil
    -- get all combinations with and without the next item:
    done += 1
    comb(pool,needed-1,done,append(chosen,done))
    comb(pool,needed,done,chosen)
end procedure

comb(5,3)