code: public Word<O> trace(final S s, final Word<I> input) {
          final FastMealyState<O> q = this.nodeToObservationMap.get(s);
          return this.observationTree.computeStateOutput(q, input);
      }
docs: Computes the output of the system under learning when applying the given input sequence in the given hypothesis
  state. Requires the input/output behavior information to be stored before.
  
  @param s
  the hypothesis state
  @param input
  the input sequence of interest
  
  @return the previously stored output behavior of the system under learning