fix issue #188

the beginner transformation `split_vc` is now using `introduce_premises`
followed by `subst_all`, instead of `simplify_trivial_quantification`
followed by `introduce_premises`.

following Andrei's suggestion, instead of `subst_all` we instead substitute
only the symbols that (1) were introduced earlier and (2) do not have
any attributes `[@model...]` so as to keep symbols present in the initial
code.