Pretty printing bigints in uxntal:
The first two bytes of the struct are the length in bytes, not limbs!
Limbs are 2 byte big-endian shorts, but limbs are stored in little-endian.
In the image, the pretty printed result of:
0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff + 1
A=1 is a bigint of length 2 bytes (that is 1 limb long) AKA a boxed short, while B is 32 bytes/256-bits (16 limbs long).
R is 17 limbs long.
https://paste.sr.ht/~ismael-vc/3e39dec521fb1c04ee598f55d25e434c1e3b229c