code: public List<WarningsGroup> getByProperty(Class<? extends ICalProperty> propertyClass) {
  		List<WarningsGroup> warnings = new ArrayList<WarningsGroup>();
  		for (WarningsGroup group : this.warnings) {
  			ICalProperty property = group.getProperty();
  			if (property == null) {
  				continue;
  			}
  
  			if (propertyClass == property.getClass()) {
  				warnings.add(group);
  			}
  		}
  		return warnings;
  	}
docs: Gets all validation warnings of a given property.
  @param propertyClass the property (e.g. {@code DateStart.class})
  @return the validation warnings