BUGFIX-66

Bug 4: Upside-Down Arithmetic Shift

• We are isolating and sign-extending a bit field.
• The bit field offsets are: at, at+1, at+2, ..., at+ct-1
• We shift the field until its highest bit is at offset 63.
• Then we shift the field until its lowest bit is at offset 0.
• The latter is an arithmetic shift to extend the sign bit.

Fix The Tiny Bug In This Go Code: