anchor.diff:              emitter.ParticleIndex <- particleIndex
               let particle = &emitter.ParticleBuffer.[particleIndex]
               particle.Life <- { LifeTimeOpt = particle.Life.LifeTimeOpt; StartTime = time }
  -            particle <- emitter.Initializer time constrain emitter
  +            particle <- emitter.ParticleInitializer time constrain emitter
   
           /// Determine emitter's liveness.
           static member getLiveness time emitter =
  
positive.code:         /// The empty constraint.
          static member empty = Constraints [||]
  
      /// The output of a particle behavior.
      type [<NoEquality; NoComparison>] Output =
          | OutputEmitter of string * Emitter
          | OutputSound of single * Sound AssetTag
  
negative.code:         static member ofSeq seq =
              Behaviors.addMany seq Behaviors.empty
  
          /// Run the behaviors over a single target.
          static member run time behaviors constrain (target : 'a) =
              let (targets, outputs) =
                  FStack.fold (fun (target, output) (behavior : Behavior) ->
                      let (targets2, output2) = behavior.Run time constrain target
                      (targets2, output + output2))
                      (target :> obj, Output.empty)
                      behaviors.Behaviors
              (targets :?> 'a, outputs)
  
          /// Run the behaviors over an array of targets.
          static member runMany time behaviors constrain (targets : 'a array) =
              let (targets, outputs) =
                  FStack.fold (fun (targets, output) (behavior : Behavior) ->
                      let (targets2, output2) = behavior.RunMany time constrain targets
                      (targets2, output + output2))
                      (targets :> obj, Output.empty)
                      behaviors.Behaviors
              (targets :?> 'a array, outputs)
  
      /// Describes an emitter.
      and [<NoEquality; NoComparison>] EmitterDescriptor<'a when 'a :> Particle and 'a : struct> =
  