When applied to a theorem A |- !x1...xn. t, the inference rule SPEC_ALL
returns the theorem A |- t[x1'/x1]...[xn'/xn] where the xi' are distinct
variants of the corresponding xi, chosen to avoid clashes with any variables
free in the assumption list and with the names of constants. Normally xi' is
just xi, in which case SPEC_ALL simply removes all universal quantifiers.
A |- !x1...xn. t
--------------------------- SPEC_ALL
A |- t[x1'/x1]...[xn'/xn]