lib/monotone.js
function Event(a, b, type, idx) {
  this.a    = a
  this.b    = b
  this.type = type
  this.idx  = idx
}
function Event(a, b, type, idx) {
  this.a    = a
  this.b    = b
  this.type = type
  this.idx  = idx
}
An event in the sweep line procedure