Thanks to both of you.

I'm emulating a 16-bit twisted ring counter--actually a very twisted counter. If an even number of bits in the top four are high, the whole works is shifted left one place and the lowest bit is set to 1; if an odd number of bits are high, again a one-place shift left, but the default 0 shifted in is retained.

Just now, the target device is a 16F87xA.