Clarify a mysterious expression

[dougm]

Summary

Replace the expression "-mask & ~mask" with a function call that does the same thing, but is commented so that it might be better understood.

[kib]

No need to resend for this style nit.

subr_blist.c

653 ↗

(On Diff #57282)

There should be a blank line after the '{' if no locals are declared.