Function interpolate

external/.d3.js:10919–10921

(from, to, direction, stream)

      notHemisphere = abs$1(cr) > epsilon$1; // TODO optimise for this common case

  function interpolate(from, to, direction, stream) {
    circleStream(stream, radius, delta, direction, from, to);
  }

  function visible(lambda, phi) {
    return cos$1(lambda) * cos$1(phi) > cr;