Problem Statement

It is defined like the Fibonacci sequence, but whereas the next term in the Fibonacci sequence is the sum of the previous two terms, in the Q sequence the previous two terms tell you how far to go back in the Q sequence to find the two numbers to sum to make the next term of the sequence.

(This point is to ensure that caching and/or recursion limits, if it is a concern, is correctly handled).

Let's start with the solution:

( scratchpad ) : next ( seq -- newseq )
dup 2 tail* over length [ swap - ] curry map
[ dupd swap nth ] map 0 [ + ] reduce suffix ;

( scratchpad ) { 1 1 } 1000 [ next ] times  dup 10 head .  999 swap nth .
{ 1 1 2 3 3 4 5 5 6 6 }
502