/// <summary>
/// Sets a group builder as the source of the negative existential element.
/// </summary>
/// <param name="builder">Element builder to set as the source.</param>
public void Group(GroupBuilder builder)
{
AssertSingleSource();
_sourceBuilder = builder;
}
/// <summary>
/// Adds a nested group builder to this group element.
/// </summary>
/// <param name="builder">Element builder to add.</param>
public void Group(GroupBuilder builder)
{
_nestedBuilders.Add(builder);
}