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    #[derive(Debug)]
    struct Child<T> {
        id: T,
        children: Vec<usize>,
    }

    impl<T> Child<T> {
        fn new(id: T) -> Self {
            Child {
                id,
                children: vec![],
            }
        }
    }

    #[derive(Debug)]
    pub(crate) struct ChildGraph<T>(Vec<Child<T>>);

    impl<T> ChildGraph<T>
    where
        T: Sized + PartialEq + Clone,
    {
        pub(crate) fn with_capacity(s: usize) -> Self {
            ChildGraph(Vec::with_capacity(s))
        }

        pub(crate) fn insert(&mut self, req: T) -> usize {
            self.0.iter().position(|e| e.id == req).unwrap_or_else(|| {
                let idx = self.0.len();
                self.0.push(Child::new(req));
                idx
            })
        }

        pub(crate) fn insert_child(&mut self, parent: usize, child: T) -> usize {
            let c_idx = self.0.len();
            self.0.push(Child::new(child));
            self.0[parent].children.push(c_idx);
            c_idx
        }

        pub(crate) fn iter(&self) -> impl Iterator<Item = &T> {
            self.0.iter().map(|r| &r.id)
        }

        pub(crate) fn contains(&self, req: &T) -> bool {
            self.0.iter().any(|r| r.id == *req)
        }
    }