Introduce ThingListBuilder::default_list

/// and should be consumed by the expression.

/// If the built value is simply a `Vec`, you can just write `built: things;`.

///

/// The `default` clause must provide an expression evaluating to a `Vec<ThingBuilder>`.  If it is

/// nontrivial, you should put the actual defaulting functionality in a (probably-private)

/// function, as the macro will expand it twice.

/// The `default` clause must provide an expression evaluating to a `Vec<ThingBuilder>`.

///

/// The `item_build` clause, if supplied, provides a closure with type

/// `FnMut(&ThingBuilder) -> Result<Thing, ConfigBuildErro>`; the default is to call

            /// Then `item` is added.

            $vis fn append(&mut self, item: $EntryBuilder) -> &mut Self {

                self.$things

                    .get_or_insert_with(|| $default)

                    .get_or_insert_with(Self::default_list)

                    .push(item);

                self

            }

                let $things = match &self.$things {

                    Some($things) => $things,

                    None => {

                        default_buffer = $default;

                        default_buffer = Self::default_list();

                        &default_buffer

                    }

                };

                    .collect::<Result<_, $crate::ConfigBuildError>>()?;

                Ok($built)

            }

            /// The default list

            fn default_list() -> Vec<$EntryBuilder> {

                 $default

            }

        }

    }

}