Add an example of how to implement a dummy package.

src/dummy_extension.rs

@@ -0,0 +1,75 @@
+use sbml_macros::{SBase, XmlWrapper};
+
+use crate::{
+    core::Model,
+    xml::{RequiredChild, RequiredProperty, XmlElement, XmlSubtype, XmlSupertype, XmlWrapper},
+};
+
+const EXTENSION_URL: &str = "http://foo";
+
+#[derive(Clone, Debug, XmlWrapper, SBase)]
+pub struct ExtendedModel(XmlElement);
+
+impl XmlSubtype<Model> for ExtendedModel {
+    fn try_cast_from_super(value: &Model) -> Option<Self> {
+        let doc = value.read_doc();
+        let declarations = value
+            .xml_element()
+            .raw_element()
+            .collect_namespace_prefixes(&doc, EXTENSION_URL);
+        if declarations.is_empty() {
+            // The extension is not declared for this model.
+            return None;
+        } else {
+            return unsafe { Some(ExtendedModel::unchecked_cast(value.clone())) };
+        }
+    }
+}
+
+impl XmlSupertype for Model {}
+
+impl ExtendedModel {
+    pub fn extra_annotation(&self) -> RequiredChild<XmlElement> {
+        RequiredChild::new(self.xml_element(), "extraAnnotation", EXTENSION_URL)
+    }
+
+    pub fn extra_property(&self) -> RequiredProperty<String> {
+        RequiredProperty::new(self.xml_element(), "extraProperty")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        dummy_extension::ExtendedModel,
+        xml::{OptionalXmlChild, RequiredXmlChild, RequiredXmlProperty, XmlSupertype, XmlWrapper},
+        Sbml,
+    };
+
+    #[test]
+    pub fn example_test() {
+        let doc = Sbml::read_str(r#"
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<sbml xmlns="http://www.sbml.org/sbml/level3/version2/core" level="3" version="2" xmlns:extra="http://foo" extra:required="false">
+    <model id="model-1" extraProperty="Some property text">
+        <extra:extraAnnotation>Some child text</extra:extraAnnotation>
+    </model>
+</sbml>
+        "#).unwrap();
+
+        let model = doc.model().get().unwrap();
+        let e_model = model.try_downcast::<ExtendedModel>().unwrap();
+        assert_eq!(
+            e_model.extra_property().get(),
+            String::from("Some property text")
+        );
+        assert_eq!(
+            e_model
+                .extra_annotation()
+                .get()
+                .xml_element()
+                .text_content(),
+            String::from("Some child text")
+        );
+    }
+}

src/lib.rs

@@ -128,6 +128,8 @@ pub(crate) mod constants;
 #[cfg(test)]
 pub mod test_suite;
 
+pub mod dummy_extension;
+
 /// The SBML container object
 /// (Section 4.1; [specification](https://raw.githubusercontent.com/combine-org/combine-specifications/main/specifications/files/sbml.level-3.version-2.core.release-2.pdf)).
 ///