The elements of prover [...] have one of the following forms, where s
is a string:

s (add s to the use-only list)
+s (add include s to the include/exclude list)
-s (add exclude s to the include/exclude list)

The include/exclude list is ordered, so that later instructions can
supersede earlier ones. The use-only list was not ordered, but now
is. The relative order of the use-only and include/exclude lists
is irrelevant, so that, e.g., prover ["Z3" +"Alt-Ergo"] and prover
[+"Alt-Ergo" "Z3"] are equivalent.

The semantics is that the use-only list is first interpreted (if it's
empty, one starts with the current provers as the base), and only then
are the instructions of the include/exclude list applied to it, in
order.

There was already the special use-only instruction "ALL". Now, there is
also the use-only instruction "CLEAR", which clears the use-only list,
but may be superseded by the use-only instructions that follow.

Examples (assuming "Z3" and "Alt-Ergo" are only known provers):
prover []. (* a no-op *)
prover [+"Z3"]  (* adds just "Z3" to whatever current provers are *)
prover [-"Z3"]  (* removes just "Z3" from whatever current provers are *)
prover ["ALL"]  (* results in "Z3", "Alt-Ergo" *)
prover ["CLEAR"]  (* results in nothing *)
prover ["CLEAR" +"Z3"]  (* results in just "Z3" *)
prover [+"Z3" "CLEAR"]  (* results in just "Z3" *)
prover ["CLEAR" "Z3"]  (* result in just "Z3" *)
prover ["Z3" "CLEAR"]  (* results in nothing *)
prover [-"Z3" "ALL"]  (* results in "Alt-Ergo" *)
prover [+"Z3" "ALL" -"Z3"]  (* results in "Alt-Ergo" *)
prover [-"Z3" "ALL" +"Z3"]  (* results in "Z3", "Alt-Ergo" *)