pub fn set<K: Into<String>, V: Into<String>>(&mut self, key: K, value: V) -> Option<String> {
        if self.values.is_none() {
            self.values = Some(BTreeMap::new());
        }
        self.values
            .as_mut()
            .unwrap()
            .insert(key.into(), value.into())
    }
Set an annotation with key associated with value.

The previous value for this key is returned if there was one.
weld/src/ast/ast.rs