code: public String toMutilLineString() {
  		final String newLine = System.getProperty("line.separator");
  		final StringBuffer problemStr = new StringBuffer();
  		for (final Problem problem : this.problems) {
  			problemStr.append(problem.getMessage() + newLine);
  		}
  		return problemStr.toString();
  	}
docs: To mutil line string.
  
  @return the string