Prevent introduce_premises from splitting too aggressively
on the left-hand side:

- under quantifiers:
    "forall x. P /\ Q" =/=> "(forall x. P) /\ (forall x. Q)"

- match-with:
    "match t with A -> P | B -> Q" =/=> "(t = A -> P) /\ (t = B -> Q)"

- if-then-else:
    "if f then P else Q" =/=> "(f -> P) /\ (not f -> Q)"

- equivalence:
    "P <-> Q" =/=> "(P -> Q) /\ (Q -> P)"

Those splits duplicate formulas, hinder readability and do not
really simplify anything. Moreover, in case of a split under
a quantifier, they create several universal premises for provers
to instantiate, instead of just one.

TODO: introduce_premises should be able to split under
a single-branch match. However, this requires special
treatment in Introduce.dequant, since the pattern
should be rather skolemized than used as a condition.