union.scala

  def union(that: TweetSet): TweetSet = {
    (left union (right union that)) incl elem
// If we have two trees x={{{.a.}b.}c{.d{.e.}}} and y={{{.f.}g.}h{.i{.j.}}} the number of calls to union varies dramatically according to how we order things:
//
//    (left union (right union that)) incl elem // calls=5
//    (right union (left union that)) incl elem // calls=5
//    (right union (that union left)) incl elem // calls=36
//    (left union (that union right)) incl elem // calls=33
//    (that union (left union right)) incl elem // calls=26
//    (that union (right union left)) incl elem // calls=26
//
//    left union right union that incl elem // calls=13
//    right union left union that incl elem // calls=13
//    right union that union left incl elem // calls=56
//    left union that union right incl elem // calls=74
//    that union left union right incl elem // calls=77
//    that union right union left incl elem // calls=90
  }