code: public void validate(String path, Schema schema, Object value, List<ValidationResult> results) {
  		String name = path != null ? path : "value";
  		boolean found = false;
  
  		for (Object thisValue : _values) {
  			if (thisValue != null && thisValue.equals(value)) {
  				found = true;
  				break;
  			}
  		}
  
  		if (!found) {
  			results.add(new ValidationResult(path, ValidationResultType.Error, "VALUE_NOT_INCLUDED",
  					name + " must be one of " + _values, _values, value));
  		}
  	}
docs: Validates a given value against this rule.
  
  @param path    a dot notation path to the value.
  @param schema  a schema this rule is called from
  @param value   a value to be validated.
  @param results a list with validation results to add new results.