AoC day 1, part 1 (dc)

[q] SX                      # exit if we run out of stack without an answer

[
    d 2020 r - * pq         # print top * (2020 - top) and exit
] SP

[
    z 0 =X                  # exit if stack empty

    d 1 r:v                 # put 1 at v[top]
    d 2020 r - ;v 1 =P      # dup, if v[2020 - top] = 1 then call Print

    s.                      # junk top of stack to remove unneeded dup
    lLx                     # loop
] SL

lLx