-- >>> let g u = vec2d ((g1 u), (g2 u))
-- >>> let u0 = vec2d (1.0, 1.0)
-- >>> let eps = 1/(10^9)
-- >>> fixed_point g eps u0
-- >>> let g u = vec2d ((g1 u), (g2 u))
-- >>> let u0 = vec2d (1.0, 1.0)
-- >>> let eps = 1/(10^9)
-- >>> fixed_point g eps u0