AlreadyBeingDifferentiated​(AngouriMath.​Entity)

Whether the rule is already partway through this very quotient. Differentiating both
parts of sqrt(x^2 - x) / x gives its reciprocal, and differentiating that gives the
first back, so the rule can go round for as long as the bound on its depth allows.
Each step of the way costs a simplification and two limits of its own, which is why
bounding the depth is not by itself enough to stop that limit running for a minute.