/**
* Adds an attribute to the name.
*
* @param string $name The attribute name. Must start with a letter and
* contain letters, digits and hyphens only.
* @param string $value The attribute value. Any non-empty string is
* allowed.
*
* @see merge()
*/
public function add($name, $value)
{
Assert::string($name, 'The attribute name must be a string. Got: %s');
Assert::notEmpty($name, 'The attribute name must not be empty.');
Assert::startsWithLetter($name, 'The attribute name %s must start with a letter.');
Assert::true((bool) preg_match('~^[a-zA-Z][a-zA-Z0-9\\-]*$~', $name), sprintf('The attribute name must contain letters, numbers and hyphens only. Got: "%s"', $name));
Assert::string($value, 'The attribute value must be a string. Got: %s');
$this->attributes[$name] = $value;
}