Generalizes the conclusion of a theorem over its own free variables.
When applied to a theorem A |- t, the inference rule GEN_ALL returns
the theorem A |- !x1...xn. t, where the xi are all the variables,
if any, which are free in t but not in the assumptions.
A |- t
A |- !x1...xn. t
Superseded by Drule.GEN_ALL, which, however, may return a different
result. That is why GEN_ALL is in hol88Lib. Sometimes people write
code that depends on the order of the quantification. They shouldn’t.