function pathArc(r, a0, a1, cx, cy) {
    return _path(null, r, a0, a1, cx, cy, 0);
}
path an arc

@param {number} r : radius
@param {number} a0 : first angular coordinate in *radians*
@param {number} a1 : second angular coordinate in *radians*
@param {number (optional)} cx : x coordinate of center
@param {number (optional)} cy : y coordinate of center
@return {string} svg path