Function subst_var 

pub fn subst_var<'db, Output, Term>(
    db: &'db dyn Db,
    bound_vars: &mut Vec<SymVariable<'db>>,
    subst_fns: &mut SubstitutionFns<'_, 'db, Term>,
    var: SymVariable<'db>,
) -> Output
where
    Term: AssertKind<'db, Output>,
    Output: FromVar<'db>,