gf_slice_set — Edition of mesh slices.
gf_slice_set(mesh_slice sl, 'pts', mat P)
    gf_slice_set(SL,('pts',mat P) : Replace the points of the slice.
        The new points P are stored in the columns the
        matrix. Note that you can use the function to apply a deformation to a
        slice, or to change the dimension of the slice (the number of rows of
        P is not required to be equal to
        gf_slice_get(SL,('dim')).