currentApproach

This is the one thing a simplification rule cannot work out for itself and the limit
machinery can: *where the expression is going*. ln(a) + ln(b) = ln(a*b) is
false off the positive reals, so Simplify may not apply it to a symbol -- but
on a stated approach the sign of each operand is decidable, and where the two signs
agree the identity is exact. Without this the rule would have to be either unsound
(as it was) or absent, and absent costs termination rather than coverage: the limit
machinery's own expansion creates logarithm pairs that only this can put back
together. https://github.com/asc-community/AngouriMath/issues/721
Thread-static because the limit machinery is synchronous and every other setting here
is; it is swapped rather than set, so a nested reading restores the outer one.