Improve iFrame tactic
- It can now also frame under later. - Better treatment of evars, it now won't end up in loops whenever the goal involves sub-formulas ?P and it trying to apply all framing rules eagerly. - It no longer delta expands while framing. - Better clean up of True sub-formulas after a successful frame. For example, framing "P" in "▷ ▷ P ★ Q" yields just "Q" instead of "▷ True ★ Q" or so.
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