/// <summary>
/// Indicates that the given sub-expression is the next expected, giving it a
/// name and specifying whether it is required.
/// </summary>
/// <param name="itemName">The name that the token will be identified with in the outer expression.</param>
/// <param name="definition">The definition for the sub-expression.</param>
/// <returns>The updated expression.</returns>
public Options Add(string itemName, ExpressionDefinition definition)
{
if (definition == null)
{
throw new ArgumentNullException("definition");
}
IExpressionItem item = new Expression(definition);
options.Add(new ExpressionItem(itemName, false, item));
return this;
}