Transient itself provides a single class for prefix commands,
transient-prefix, but package authors may wish to define specialized
classes. Doing so makes it possible to change the behavior of the set
of prefix commands that use that class, by implementing specialized
methods for certain generic functions (see Prefix Methods).
A transient prefix command’s object is stored in the transient--prefix
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.