def members(SWcoord, NEcoord, particles):
    """
    Helper function:
    Return the sub-list of particles that exist in the
    box defined by SWcoord and NE coord.

    Parameters
    ----------
    SWcoord: Coordinate
        Coordinate of the SW corner of the box
    NEcoord : Coordinate
        Coordinate of the NE corner of the box
    particles : list
        list of Particles to search membership over

    Returns
    -------
    retval : list
        list of particles existing in the region
    """
    retval = list()

    for particle in particles:
        c = particle.get_coord()
        # Using the operator overload logic defined in Coordinate
        if SWcoord <= c and c <= NEcoord:
            retval.append(particle)

    return retval