HP-42S (Free42, DM-42) mini-challenge – Wallis Product

[Werner]

The limit of my routine on Free42 is 10203, which is quite a lot larger though.
It could've been slightly larger still, but COMB errors out on C(20408,10204) even though the result is less than 1e6145. COMB should be able to compute up to C(20420,10210) using a little trick, as demonstrated in this RPN version:

Code:

00 { 71-Byte Prgm }
01▸LBL "CMB"
02 LSTO "L"
03 R↓
04 R↓
05 LSTO "Z"
06 R↓
07 LSTO "T"
08 R↓
09 STO- ST Y
10 X>Y?
11 X<>Y
12 STO+ ST Y
13 1
14 X>Y?
15 GTO 00
16 1ᴇ-4
17▸LBL 02
18 RCL× ST T
19 RCL÷ ST Y
20 DSE ST T
21 ISG ST Y
22 X<>Y
23 DSE ST Z
24 GTO 02
25 1ᴇ4
26 ×
27▸LBL 00
28 RCL "L"
29 ABS
30 RCL "T"
31 STO ST Y
32 RCL "Z"
33 R↑
34 END

Cheers, Werner