UnsolvedWhereIndependenceIsDenied​(AngouriMath.​Entity.​Set,​AngouriMath.​Entity,​AngouriMath.​Entity.​Variable)

derivative(y, x) + y - x was answered { x }: the derivative went to
zero because y is not x, and the root then says that it is. Putting
it back gives derivative(x, x) + x - x, which is 1 — so the set named a
member that is not a root. A root free of that name is untouched, because nothing
was assumed that it goes on to deny: derivative(y * x, x) + y - 1 is still
{ 1/2 }.

The equation is not thereby unsatisfiable, so the empty set would be a second
false claim in place of the first. What holds is the condition as written, and
solving it needs a differential-equation solver this library does not have.
#964,
#746