The clev field in the node struct is almost always multiplied by WIDTH; occasionally, it is incremented and then multiplied by WIDTH. Instructions can be saved by storing it always multiplied by WIDTH.
For the computation of slot(), this just eliminates a multiplication. For trimkey(), where the caller always adds one to clev before passing it as an argument, this means having the caller not do that, and having trimkey handle it not by adding WIDTH to the input parameter, but by having it shift COUNT, and not 1. That leads to the same result when level is small, and it relieves keybarr of the need to test to avoid shifting by a possibly-too-large value, since level is always < 64.