code: public Set<String> getSearchIDs(String problemID){
          if(!results.containsKey(problemID)){
              throw new UnknownIDException("Unknown problem ID " + problemID + ".");
          }
          return Collections.unmodifiableSet(results.get(problemID).keySet());
      }
docs: Get the IDs of the different searches that have been applied to solve the problem with the
  given ID (unmodifiable view).
  
  @param problemID ID of the problem
  @return IDs of different searches applied to solve the problem
  @throws UnknownIDException if an unknown problem ID is given