It boils down to: > (* 4294967296 4294967296) $1 = 0 My guess is that guile uses the native int type to store small numbers, and then switches to a bignums,

It boils down to: > (* 4294967296 4294967296) $1 = 0 My guess is that guile uses the native int type to store small numbers, and then switches to a bignums, backed by GNU MP when the native ints are too small. Maybe in that particular case, the check fails, and the computation overflows the native int.