F'(g(x)) = [ F(g(x+dx)) - F(g(x)) ] / dx .(1)
g(x+dx) - g(x) = g'(x)*dx = dg(x) .(2)
g(x+dx) = g(x) + dg(x) .(3)
F'(g(x)) = [ F(g(x) + dg(x)) - F(g(x)) ] /dx =
[ F(g(x) + dg(x)) - F(g(x)) ] / dg(x) * dg(x)/dx =
F'(g) * g'(x)