transient-prefix class is being used for all prefix
command and there is only a single generic functions that can be
specialized based on the class of a prefix command.
This generic function is called while setting up the transient and
is responsible for initializing the
history slot. This is the
transient-wide history; many individual infixes also have a history
of their own.
The default (and currently only) method extracts the value from the
A transient prefix command’s object is stored in the
property of the command symbol. While a transient is active, a clone
of that object is stored in the variable
transient--prefix. A clone
is used because some changes that are made to the active transient’s
object should not affect later invocations.