function link() {
  // Exposed variables
  var _ = { source : function(d) { return d.source; },
            target : function(d) { return d.target; },
            angle  : function(d) { return d.angle; },
            startRadius : function(d) { return d.radius; },
            endRadius : function(d) { return d.radius; },
            arcOffset : -Math.PI / 2
          };
          
  d3.expose.call(link,_)

  function link(d, i) {
    var s = node(_.source, this, d, i),
        t = node(_.target, this, d, i),
        x;
    if (t.a < s.a) x = t, t = s, s = x;
    if (t.a - s.a > Math.PI) s.a += 2 * Math.PI;
    var a1 = s.a + (t.a - s.a) / 3,
        a2 = t.a - (t.a - s.a) / 3;
    return s.r0 - s.r1 || t.r0 - t.r1
        ? "M" + Math.cos(s.a) * s.r0 + "," + Math.sin(s.a) * s.r0
        + "L" + Math.cos(s.a) * s.r1 + "," + Math.sin(s.a) * s.r1
        + "C" + Math.cos(a1) * s.r1 + "," + Math.sin(a1) * s.r1
        + " " + Math.cos(a2) * t.r1 + "," + Math.sin(a2) * t.r1
        + " " + Math.cos(t.a) * t.r1 + "," + Math.sin(t.a) * t.r1
        + "L" + Math.cos(t.a) * t.r0 + "," + Math.sin(t.a) * t.r0
        + "C" + Math.cos(a2) * t.r0 + "," + Math.sin(a2) * t.r0
        + " " + Math.cos(a1) * s.r0 + "," + Math.sin(a1) * s.r0
        + " " + Math.cos(s.a) * s.r0 + "," + Math.sin(s.a) * s.r0
        : "M" + Math.cos(s.a) * s.r0 + "," + Math.sin(s.a) * s.r0
        + "C" + Math.cos(a1) * s.r1 + "," + Math.sin(a1) * s.r1
        + " " + Math.cos(a2) * t.r1 + "," + Math.sin(a2) * t.r1
        + " " + Math.cos(t.a) * t.r1 + "," + Math.sin(t.a) * t.r1;
  }

  function node(method, thiz, d, i) {
    var node = method.call(thiz, d, i),
        a = +(typeof _.angle === "function" ? _.angle.call(thiz, node, i) : _.angle) + _.arcOffset,
        r0 = +(typeof _.startRadius === "function" ? _.startRadius.call(thiz, node, i) : _.startRadius),
        r1 = (_.startRadius === _.endRadius ? r0 : +(typeof _.endRadius === "function" ? _.endRadius.call(thiz, node, i) : _.endRadius));
    return {r0: r0, r1: r1, a: a};
  }

  return link;
};


// Standard getter/setter
d3.expose = function(_) { 
  var self = this 
  Object.keys(_).forEach(function(key) { 
    self[key] = function(x) { 
      if (!arguments.length) return _[key]; 
      _[key] = x; 
      return self; 
    } 
  }); 
}