improve syntactic characterization to take simplification rules into account
!((G(X(p0))) & (F(G(!(p1)))))
is a syntactic recurrence, but it simplifies to XF(!p0 | GFp1)
which is not detected as such.
!((G(X(p0))) & (F(G(!(p1)))))
is a syntactic recurrence, but it simplifies to XF(!p0 | GFp1)
which is not detected as such.