message: Add context lookup test
diff: @@ -139,6 +139,7 @@ impl<'a> Context<'a> {
  #[cfg(test)]
  mod tests {
  use super::*;
  + use core::SValue;
  fn parse(src: &str) -> ITerm {
  use parse::Term;
  @@ -146,6 +147,27 @@ mod tests {
  Term::to_core(&src.parse().unwrap()).unwrap()
  }
  + #[test]
  + fn extend_lookup() {
  + let x = Rc::new(Value::from(SValue::Var(Var::Free(Name(String::from("x"))))));
  + let y = Rc::new(Value::from(SValue::Var(Var::Free(Name(String::from("y"))))));
  +
  + let context0 = Context::Nil;
  +
  + assert_eq!(context0.lookup(Debruijn(0)), None);
  +
  + let context1 = context0.extend(x.clone());
  +
  + assert_eq!(context1.lookup(Debruijn(0)), Some(&x));
  + assert_eq!(context1.lookup(Debruijn(1)), None);
  +
  + let context2 = context1.extend(y.clone());
  +
  + assert_eq!(context2.lookup(Debruijn(0)), Some(&y));
  + assert_eq!(context2.lookup(Debruijn(1)), Some(&x));
  + assert_eq!(context2.lookup(Debruijn(2)), None);
  + }
  +
  mod infer {
  use super::*;
  