def qr(X, ip_B=None, reorthos=1):
    """QR factorization with customizable inner product.

    :param X: array with ``shape==(N,k)``
    :param ip_B: (optional) inner product, see :py:meth:`inner`.
    :param reorthos: (optional) numer of reorthogonalizations. Defaults to
      1 (i.e. 2 runs of modified Gram-Schmidt) which should be enough in most
      cases (TODO: add reference).

    :return: Q, R where :math:`X=QR` with :math:`\\langle Q,Q \\rangle=I_k` and
      R upper triangular.
    """
    if ip_B is None and X.shape[1] > 0:
        return scipy.linalg.qr(X, mode='economic')
    else:
        (N, k) = X.shape
        Q = X.copy()
        R = numpy.zeros((k, k), dtype=X.dtype)
        for i in range(k):
            for reortho in range(reorthos+1):
                for j in range(i):
                    alpha = inner(Q[:, [j]], Q[:, [i]], ip_B=ip_B)[0, 0]
                    R[j, i] += alpha
                    Q[:, [i]] -= alpha * Q[:, [j]]
            R[i, i] = norm(Q[:, [i]], ip_B=ip_B)
            if R[i, i] >= 1e-15:
                Q[:, [i]] /= R[i, i]
        return Q, R