code: public Problem getProblem(final String letter) {
  		if (letter.isEmpty() || letter.length() > 1) {
  			return null;
  		}
  		final char identifier = letter.toUpperCase().charAt(0);
  		final int index = (int) identifier - (int) 'A';
  		final List<Problem> problems = info.getProblems();
  		if (index < 0 || index >= problems.size()) {
  			return null;
  		}
  		return problems.get(index);
  	}
docs: <p>Retrieves the problem associated
  to the given <tt>letter</tt>.</p>
  
  @param letter Letter that identifies the problem.
  @return Corresponding problem if exist, <tt>null</tt> otherwise.