def _is_a_vertex_of_polygon(x, y, polygon):
    """
    Check if the `x`/`y` coordinate is a vertex of the `polygon`.

        >>> polygon = ((0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 1.0))

        >>> _is_a_vertex_of_polygon(0.0, 0.0, polygon)
        True

        >>> _is_a_vertex_of_polygon(0.5, 0.5, polygon)
        False

    Beware:
        - latitude is `y` and longitude is `x`
        - coordinate and `polygon` must use the same geodesic projection system
    """
    return (x, y) in polygon