--- gcc compile output (empty = zero warnings) ---
(compiled clean)
--- spot values ---
f(0  ) python=0 c=0
f(1  ) python=1 c=1
f(2  ) python=1 c=1
f(10 ) python=55 c=55
f(50 ) python=12586269025 c=12586269025
f(100) python=354224848179261915075 c=354224848179261915075
--- exact diff over many N ---
ALL MATCH
--- f(1000) via C (209 digits, beyond 64-bit) ---
43466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875
