anchor.diff:                Body = { Position = v2Zero; LinearVelocity = v2Zero; Rotation = 0.0f; AngularVelocity = 0.0f; Gravity = v2 0.0f -9.81f }
                 Image = image
                 Rate = rate
  -              Id = Gen.id }+              Id = Gen.id }
  
positive.code:             member this.Run time constrain =
                  let (emitter, output) = Emitter<'a>.run time constrain this
                  (emitter :> Emitter, output)
              end
  
      /// A particle system.
  
negative.code:     /// making it harder to use this existing emitter as an example.
      type [<NoEquality; NoComparison>] Emitter<'a when 'a :> Particle and 'a : struct> =
          { Life : Life
            ParticleLifeTimeOpt : int64 // OPTIMIZATION: uses 0L to represent infinite particle life.
            mutable ParticleIndex : int // operates as a ring-buffer
            mutable ParticleWatermark : int // tracks the highest active particle index; never decreases.
            Particles : 'a array // operates as a ring-buffer
  