SolveByCancellingInsideALogarithm​(AngouriMath.​Entity,​AngouriMath.​Entity.​Variable,​System.​Boolean)

A logarithm whose argument is a quotient in x that cancels with
the functions of x in it taken for indeterminates, with the
argument cancelled. That is the spelling the parser gives an inverse hyperbolic
function of a hyperbolic one -- atanh(tanh(u)) is
1/2 ln((1 + T)/(1 - T)) with T = (E - 1)/(E + 1) and E = e^(2u),
which as one quotient is 2E(E + 1)/(2(E + 1)), and is 1/2 ln(E): an
atom whose derivative is u', which every rule downstream reads, where the
nested quotient was read by none. acoth(tanh(u)) is 1/2 ln(-E) the
same way. Kept as the logarithm rather than unwrapped to u, since
ln(e^(2u)) is 2u only on a strip, and the logarithm is exact
everywhere -- the answer carries it as Rubi's carries atanh(tanh(u)).
Rubi's 7.3.7 and 7.4.1. Asked as a question of its own.
https://github.com/asc-community/AngouriMath/issues/718