improves the
    rate of convergence, which implies that fewer iterations are needed
    to reach a given error tolerance.
callback : function
    User-supplied function to call after each iteration.  It is called
    as ``callback(xk)``, where ``xk`` is the current solution vector.

Returns
-------
x : ndarray
    The converged solution.
info : integer
    Provides convergence information:
        0  : successful exit
        >0 : convergence to tolerance not achieved, number of iterations
        <0 : parameter breakdown

Notes
-----
The preconditioner `M` should be a matrix such that ``M @ A`` has a smaller
condition number than `A`, see [1]_ .

References
----------
.. [1] "Preconditioner", Wikipedia, 
       https://en.wikipedia.org/wiki/Preconditioner
.. [2] "Biconjugate gradient stabilized method", 
       Wikipedia, https://en.wikipedia.org/wiki/Biconjugate_gradient_stabilized_method

Examples
--------
>>> import numpy as np
>>> from scipy.sparse import csc_array
>>> from scipy.sparse.linalg import bicgstab
>>> R = np.array([[4, 2, 0, 1],
...               [3, 0, 0, 2],
...               [0, 1, 1, 1],
...               [0, 2, 1, 0]])
>>> A = csc_array(R)
>>> b = np.array([-1, -0.5, -1, 2])
>>> x, exit_code = bicgstab(A, b, atol=1e-5)
>>> print(exit_code)  # 0 indicates successful convergence
0
>>> np.allclose(A.dot(x), b)
True
r