Function minitt::type_check::update_gamma

pub fn update_gamma<'a, Name: DebuggableNameTrait>(
    gamma: &'a mut BTreeMap<Name, Value<Name>>, 
    pattern: &Pattern<Name>, 
    type_val: Value<Name>, 
    val: Value<Name>
) -> Result<&'a mut BTreeMap<Name, Value<Name>>, String>

upG in Mini-TT. Gamma |- p : t = u => Gamma’

However, since Rust is an imperative language, we use mutable reference instead of making it monadic.