I (tgl) objected to the obscure implementation introduced in commit
1c497fa7.  This one seems a bit less action-at-a-distance-y, at the
price of repeating a few lines of code.

Improve the comments about what the function is doing, too.

Amit Khandekar, whacked around a bit more by me

Discussion: https://postgr.es/m/CAJ3gD9egYTyHUH0nTMxm8-1m3RvdqEbaTyGC-CUNtYf7tKNDaQ@mail.gmail.com