def gen_layer_circle(radius, sort_key, fill_color, fill_opacity,
    stroke_color, stroke_width):
    """
    Build a Mapbox GL Style layer for circle.
    """
    return {'type': 'circle', 'layout': {'circle-sort-key': ['-', sort_key]
        }, 'paint': {'circle-radius': radius, 'circle-color': fill_color,
        'circle-opacity': fill_opacity, 'circle-stroke-color': stroke_color,
        'circle-stroke-width': stroke_width}}
