allow unabbreviation before converting formulas in ltldo and ltlcross

% ltldo modella -f 'p0 -> p1'
modella: syntax error: p0
error: execution of command "modella 'lcr-i1-hXpGvq' 'lcr-o0-XpHwfQ'" returned exit code 255.
ltldo: aborting here

This is because MoDeLLa uses the LBT syntax but does not support i (implication) e (equivalence) or ^ (xor).

We have similar issues with Rabinizer3 that supports U but not its dual R. It would be nice if the syntax for specifying translators could support a way to list operators that should be rewritten.

I'm thinking about %[MWie^]L for MoDeLLa and %[RMW]ffor Rabinizer3. The custom syntax for Python's format() should support this as well.