When applied to a theorem A |- !x1...xn. t, where the number of universally
quantified variables may be zero, GSPEC returns A |- t[g1/x1]...[gn/xn],
where the gi are distinct variable names of the appropriate type, chosen by
genvar.
        A |- !x1...xn. t
   -------------------------  GSPEC
    A |- t[g1/x1]...[gn/xn]