interpolate.source = function(_) {
      var cx0 = Math.cos(x0 = _[0] * d3_geo_radians), sx0 = Math.sin(x0);
      cy0 = Math.cos(y0 = _[1] * d3_geo_radians);
      sy0 = Math.sin(y0);
      kx0 = cy0 * cx0;