Function store_var 

pub fn store_var<BeforeOverwrite, AfterStore>(
    stack: &mut Vec<Value>,
    vars: &mut Vec<Value>,
    index: usize,
    var_names: &HashMap<usize, String>,
    before_overwrite: BeforeOverwrite,
    after_store: AfterStore,
) -> Result<(), RuntimeError>
where
    BeforeOverwrite: FnMut(&Value, &Value),
    AfterStore: FnMut(usize, &Value),