================================== FAILURES ==================================
_____________ [doctest] mpmath.calculus.optimization.steffensen ______________
1034 
1035     x* is a fixpoint of the iteration x_{k+1} = phi(x_k) if x* = phi(x*). For
1036     phi(x) = x**2 there are two fixpoints: 0 and 1.
1037 
1038     Let's try Steffensen's method:
1039 
1040     >>> f = lambda x: x**2
1041     >>> from mpmath.calculus.optimization import steffensen
1042     >>> F = steffensen(f)
1043     >>> for x in [0.5, 0.9, 2.0]:
Differences (unified diff with -expected +actual):
    @@ -5,5 +5,5 @@
              2.32831e-10          -2.84465e-27
              5.42101e-20           2.30189e-80
    -         2.93874e-39          -1.2197e-239
    +         2.93874e-39                     0
              8.63617e-78                     0
             7.45834e-155                     0

/var/tmp/portage/dev-python/mpmath-1.4.0_alpha1/work/mpmath-1.4.0a1/mpmath/calculus/optimization.py:1043: DocTestFailure