code: def mark_the_spot(mask):
      """Marks every nonzero voxel using streamlines to form a 3D 'X' inside.
  
      Generates streamlines forming a 3D 'X' inside every nonzero voxel.
  
      Parameters
      ----------
      mask : ndarray
          Mask containing the spots to be marked.
  
      Returns
      -------
      list of ndarrays
          All streamlines needed to mark every nonzero voxel in the `mask`.
      """
  
      def _gen_straight_streamline(start, end, steps=3):
          coords = []
          for s, e in zip(start, end):
              coords.append(np.linspace(s, e, steps))
  
          return np.array(coords).T
  
      # Generate a 3D 'X' template fitting inside the voxel centered at (0,0,0).
      X = [
          _gen_straight_streamline((-0.5, -0.5, -0.5), (0.5, 0.5, 0.5)),
          _gen_straight_streamline((-0.5, 0.5, -0.5), (0.5, -0.5, 0.5)),
          _gen_straight_streamline((-0.5, 0.5, 0.5), (0.5, -0.5, -0.5)),
          _gen_straight_streamline((-0.5, -0.5, 0.5), (0.5, 0.5, -0.5)),
      ]
  
      # Get the coordinates of voxels 'on' in the mask.
      coords = np.array(zip(*np.where(mask)))
  
      streamlines = [
          (line + c) * voxel_size for c in coords for line in X
      ]
  
      return streamlines
  
imports: import numpy
  import nibabel
  from nibabel.streamlines import FORMATS
  from nibabel.streamlines.header import Field
label_desc: Use image node_id 1 for calling a global function with example usage: mark_the_spot(mask) and returns: streamlines, np