{"ast_node_type":"other","tokens":[{"end":26,"kind":"keyword","start":0,"text":"ProofTreeBuilder"}]}
        ProofTreeBuilder {
    fn opt_nested<T: Into<DebugSolver<I>>>(&self, state: impl FnOnce() -> Option<T>) -> Self {
            state: self.state.as_ref().and_then(|_| Some(state()?.into())).map(Box::new),