Prevent a recurrence of #20103
On legacy/trac#20103 (moved), Roger sensibly asks:
Assuming for the moment that it is the right patch though: are there things we should do to remove this trap for future developers? Maybe a huge comment would be an easy first step? And maybe "precompute the answer to what that macro was about, and locate where in the code the answer might change, and only change it then" as another step?