function path(d, i) {
    if (typeof pointRadius === "function") pointCircle = d3_path_circle(pointRadius.apply(this, arguments));
    pathType(d);
    var result = buffer.length ? buffer.join("") : null;
    buffer = [];
    return result;
  }
Returns a function that, given a GeoJSON object (e.g., a feature), returns
the corresponding SVG path. The function can be customized by overriding the
projection. Point features are mapped to circles with a default radius of
4.5px; the radius can be specified either as a constant or a function that
is evaluated per object.