code: @NonNull
      public TransitionSet addTransition(@Nullable Transition transition) {
          if (transition != null) {
              addTransitionInternal(transition);
              if (mDuration >= 0) {
                  transition.setDuration(mDuration);
              }
              if (mInterpolator != null) {
                  transition.setInterpolator(mInterpolator);
              }
          }
          return this;
      }
docs: Adds child transition to this set. The order in which this child transition
  is added relative to other child transitions that are added, in addition to
  the {@link #getOrdering() ordering} property, determines the
  order in which the transitions are started.
  <p/>
  <p>If this transitionSet has a {@link #getDuration() duration} set on it, the
  child transition will inherit that duration. Transitions are assumed to have
  a maximum of one transitionSet parent.</p>
  
  @param transition A non-null child transition to be added to this set.
  @return This transitionSet object.