[parser] Add a single pair test

parser/tests/single_item.rs

@@ -11,7 +11,7 @@ extern crate sibiltypes;
 use sibillexer::{Lex, Token};
 use sibillexer::Result as LexerResult;
 use sibilparser::Parser;
-use sibiltypes::{Obj, Sym};
+use sibiltypes::{Obj, Pair, Sym};
 
 #[test]
 fn single_sym() {
@@ -21,3 +21,12 @@ fn single_sym() {
     assert_eq!(parser.next(), Some(Ok(Obj::new(Sym::with_str("abc")))));
     assert_eq!(parser.next(), None);
 }
+
+#[test]
+fn single_pair() {
+    let tokens = vec![Ok(Lex::new(Token::LeftParen, "(", 0, 0)),
+                      Ok(Lex::new(Token::RightParen, ")", 0, 0))].into_iter();
+    let mut parser = Parser::new(tokens);
+    assert_eq!(parser.next(), Some(Ok(Obj::new(Pair::empty()))));
+    assert_eq!(parser.next(), None);
+}