The previous comment mis-stated the number of bits in mPi4.
The correct value is 19*64 + 1 == 1217 bits.

Change-Id: Ife971ff6936ce2d5b81ce663ce48044749d592a0
Reviewed-on: https://go-review.googlesource.com/c/154017Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>