/// Expander - A callback function that can transform a Record representing a
/// set into a fully expanded list of elements. Expanders provide a way for
/// users to define named sets that can be used in DAG expressions.
/// Expander - A callback function that can transform a Record representing a
/// set into a fully expanded list of elements. Expanders provide a way for
/// users to define named sets that can be used in DAG expressions.