What happens:
testfor returns true when rxm <= player_rotation <= rx

What should happen:
if rxm <= rx then
(returns true when rxm <= player_rotation <= rx)
else
(returns true when rxm <= player_rotation OR player_rotation <= rx)

With this change, it is appropriate to rename rxm and rx from minimum/maximum x rotation to leftmost and rightmost angle.

This change will not alter any outputs for current usage where rxm <= rx

This will allow arbitrary sectors to be used for testing, regardless of whether the sector intersects with the discontinuity in the north direction. This will allow me to use less red stone circuitry when testing if I am looking in the north direction from 110 to -110. Currently I would have to use two command blocks to split up the sector: -180 to -110 and 110 to 180.

A similar change may be made with the y rotation which would allow an arbitrary vertical angle to wrap from the bottom to top. However, since the mechanics has a discontinuity rather than just the model, it would be more difficult to intuitively describe the bounds in documentation than it would be for the x rotation.