def METHOD_NAME(ranges_i, ranges_j, keep):
    r"""Turns a boolean matrix into a KeOps-friendly **ranges** argument.
    This routine is a helper for the **block-sparse** reduction mode of KeOps,
    allowing you to turn clustering information (**ranges_i**,
    **ranges_j**) and a cluster-to-cluster boolean mask (**keep**)
    into integer tensors of indices that can be used to schedule the KeOps routines.
    Suppose that you're working with variables :math:`x_i`  (:math:`i \in [0,10^6)`),
    :math:`y_j`  (:math:`j \in [0,10^7)`), and that you want to compute a KeOps reduction
    over indices :math:`i` or :math:`j`: Instead of performing the full
    kernel dot product (:math:`10^6 \cdot 10^7 = 10^{13}` operations!),
    you may want to restrict yourself to
    interactions between points :math:`x_i` and :math:`y_j` that are "close" to each other.
    With KeOps, the simplest way of doing so is to:
    1. Compute cluster labels for the :math:`x_i`'s and :math:`y_j`'s, using e.g.
       the :func:`grid_cluster` method.
    2. Compute the ranges (**ranges_i**, **ranges_j**) and centroids associated
       to each cluster, using e.g. the :func:`cluster_ranges_centroids` method.
    3. Sort the tensors ``x_i`` and ``y_j`` with :func:`sort_clusters` to make sure that the
       clusters are stored contiguously in memory (this step is **critical** for performance on GPUs).
    At this point:
        - the :math:`k`-th cluster of :math:`x_i`'s is given by ``x_i[ ranges_i[k,0]:ranges_i[k,1], : ]``, for :math:`k \in [0,M)`,
        - the :math:`\ell`-th cluster of :math:`y_j`'s is given by ``y_j[ ranges_j[l,0]:ranges_j[l,1], : ]``, for :math:`\ell \in [0,N)`.
    4. Compute the :math:`(M,N)` matrix **dist** of pairwise distances between cluster centroids.
    5. Apply a threshold on **dist** to generate a boolean matrix ``keep = dist < threshold``.
    6. Define a KeOps reduction ``my_genred = Genred(..., axis = 0 or 1)``, as usual.
    7. Compute the block-sparse reduction through
       ``result = my_genred(x_i, y_j, ranges = from_matrix(ranges_i,ranges_j,keep) )``
    :func:`from_matrix` is thus the routine that turns a **high-level description**
    of your block-sparse computation (cluster ranges + boolean matrix)
    into a set of **integer tensors** (the **ranges** optional argument),
    used by KeOps to schedule computations on the GPU.
    Args:
        ranges_i ((M,2) integer array): List of :math:`[\text{start}_k,\text{end}_k)` indices.
            For :math:`k \in [0,M)`, the :math:`k`-th cluster of ":math:`i`" variables is
            given by ``x_i[ ranges_i[k,0]:ranges_i[k,1], : ]``, etc.
        ranges_j ((N,2) integer array): List of :math:`[\text{start}_\ell,\text{end}_\ell)` indices.
            For :math:`\ell \in [0,N)`, the :math:`\ell`-th cluster of ":math:`j`" variables is
            given by ``y_j[ ranges_j[l,0]:ranges_j[l,1], : ]``, etc.
        keep ((M,N) boolean array):
            If the output ``ranges`` of :func:`from_matrix` is used in a KeOps reduction,
            we will only compute and reduce the terms associated to pairs of "points"
            :math:`x_i`, :math:`y_j` in clusters :math:`k` and :math:`\ell`
            if ``keep[k,l] == 1``.
    Returns:
        A 6-uple of integer arrays that can be used as an optional **ranges**
        argument of :func:`Genred <pykeops.numpy.Genred>`. See the documentation of :func:`Genred <pykeops.numpy.Genred>` for reference.
    Example:
        >>> r_i = np.array( [ [2,5], [7,12] ], dtype=int )          # 2 clusters: X[0] = x_i[2:5], X[1] = x_i[7:12]
        >>> r_j = np.array( [ [1,4], [4,9], [20,30] ], dtype=int )  # 3 clusters: Y[0] = y_j[1:4], Y[1] = y_j[4:9], Y[2] = y_j[20:30]
        >>> x,y = np.array([1., 0.]), np.array([1.5, .5, 2.5])      # dummy "centroids"
        >>> dist = (x[:,None] - y[None,:])**2
        >>> keep = (dist <= 1)                                  # (2,3) matrix
        >>> print(keep)
        [[ True  True False]
         [False  True False]]
        --> X[0] interacts with Y[0] and Y[1], X[1] interacts with Y[1]
        >>> (ranges_i,slices_i,redranges_j, ranges_j,slices_j,redranges_i) = from_matrix(r_i,r_j,keep)
        --> (ranges_i,slices_i,redranges_j) will be used for reductions with respect to "j" (axis=1)
        --> (ranges_j,slices_j,redranges_i) will be used for reductions with respect to "i" (axis=0)
        Information relevant if **axis** = 1:
        >>> print(ranges_i)  # = r_i
        [[ 2,  5],
         [ 7, 12]]
        --> Two "target" clusters in a reduction wrt. j
        >>> print(slices_i)
        [2, 3]
        --> X[0] is associated to redranges_j[0:2]
        --> X[1] is associated to redranges_j[2:3]
        >>> print(redranges_j)
        [[1, 4],
         [4, 9],
         [4, 9]]
        --> For X[0], i in [2,3,4],       we'll reduce over j in [1,2,3] and [4,5,6,7,8]
        --> For X[1], i in [7,8,9,10,11], we'll reduce over j in [4,5,6,7,8]
        Information relevant if **axis** = 0:
        >>> print(ranges_j)
        [[ 1,  4],
         [ 4,  9],
         [20, 30]]
        --> Three "target" clusters in a reduction wrt. i
        >>> print(slices_j)
        [1, 3, 3]
        --> Y[0] is associated to redranges_i[0:1]
        --> Y[1] is associated to redranges_i[1:3]
        --> Y[2] is associated to redranges_i[3:3] = no one...
        >>> print(redranges_i)
        [[ 2,  5],
         [ 2,  5],
         [ 7, 12]]
        --> For Y[0], j in [1,2,3],     we'll reduce over i in [2,3,4]
        --> For Y[1], j in [4,5,6,7,8], we'll reduce over i in [2,3,4] and [7,8,9,10,11]
        --> For Y[2], j in [20,21,...,29], there is no reduction to be done
    """
    J, I = np.meshgrid(np.arange(0, keep.shape[1]), np.arange(0, keep.shape[0]))
    redranges_i = ranges_i[
        I.T[keep.T]
    ]  # Use PyTorch indexing to "stack" copies of ranges_i[...]
    redranges_j = ranges_j[J[keep]]
    slices_i = np.cumsum(
        np.sum(keep, axis=1), axis=0
    )  # slice indices in the "stacked" array redranges_j
    slices_j = np.cumsum(
        np.sum(keep, axis=0), axis=0
    )  # slice indices in the "stacked" array redranges_i
    return (
        ranges_i.astype("int32"),
        slices_i.astype("int32"),
        redranges_j.astype("int32"),
        ranges_j.astype("int32"),
        slices_j.astype("int32"),
        redranges_i.astype("int32"),
    )