Hard to resolve error "set: Cannot mutate immutable Rc articulation; use an DCG cell instead"

[Cypher1]

Attempting to get a test working with the following code.

Not sure how to get the re-use and incremental compute working and I'm unsure of what the error message means (this is my first time using adapton).

Code sample:

    #[test]
    fn test_adapton_parser_pair() -> Result<(), TError> {
        use adapton::*;
        use std::sync::{Arc, Mutex};

        let calls = Arc::new(Mutex::new(0));
        let counter = calls.clone();
        let input = cell!([input] "1,2");
        let ast = thunk!([ast]
            |src: &str| {
                *counter.lock().unwrap() += 1;
                setup(src)
            };
            src: get!(input)
        );
        {
            set(&input, "1,2");
            let ast = get!(ast)?;
            assert_eq!(ast.identifiers.len(), 0);
            assert_eq!(ast.literals.len(), 2);
            assert_eq!(ast.ops.len(), 1);
            assert_eq!(ast.definitions.len(), 0);
            assert_eq!(*calls.lock().unwrap(), 1);
        }
        {
            set(&input, "1,2,3");
            let ast = get!(ast)?;
            assert_eq!(ast.identifiers.len(), 0);
            assert_eq!(ast.literals.len(), 3);
            assert_eq!(ast.ops.len(), 2);
            assert_eq!(ast.definitions.len(), 0);
            assert_eq!(*calls.lock().unwrap(), 2);
        }
        {
            set(&input, "1,2");
            let ast = get!(ast)?;
            assert_eq!(ast.identifiers.len(), 0);
            assert_eq!(ast.literals.len(), 2);
            assert_eq!(ast.ops.len(), 1);
            assert_eq!(ast.definitions.len(), 0);
            assert_eq!(*calls.lock().unwrap(), 2);
        }
        Ok(())
    }

Would love some help with this as I'm fairly certain that I'm following the Adapton documentation (here: https://docs.rs/adapton/0.3.31/adapton/ ) fairly closely.

Thanks