Achim Gratz writes:
[…]
> in the second or the cdr of the second element of pre-info might
> directly get the new value spliced in depending on whether the original
> value is used someplace else.

Splicing seems slightly more elegant than list construction, but
pre-info needs to be preserved.  Eric, please review the attached patch,
I'm not certain about the current test coverage in that area.