twee-2.3: tests/y-easy.p

fof(k_def, axiom, ![X, Y]: (k @ X) @ Y = X).
fof(s_def, axiom, ![X, Y, Z]: ((s @ X) @ Y) @ Z = (X @ Z) @ (Y @ Z)).
fof(conjecture, conjecture, ![F]: ?[X]: F @ X = X).