Circular reasoning with implicit functions

[gdalle]

While working on #980 I noticed something strange in your use of ImplicitDifferentiation. You seem to be using a circular reasoning like the following:

forward(x) = implicit(x)
conditions(x, y, z) = ...
const implicit = ImplicitFunction(forward, conditions)

Can you explain why this is necessary? Higher-order AD perhaps?

[baggepinnen]

Which set of methods in particular are circular? I think they all accept different type signatures or have subtly different names, e.g., are, arec, ared etc., otherwise the tests would have failed with stack overflow?