code: function( arr ){
  
              if ( arr === true ){
                  this._targets = null;
              } else {
                  this._targets = Physics.util.uniq( arr );
              }
              return this;
          }
docs: Behavior#applyTo( arr ) -> this
  - arr (Array): Array of bodies to apply this behavior to. Specify `true` for all objects in world.
  
  Apply the behavior to a group of bodies.