public void CanDeleteCurrentEntry()
 {
     var engine = new Engine();
       engine.Digit(1);
       Assert.AreEqual(1, engine.Value);
       engine.ClearEntry();
       Assert.AreEqual(0, engine.Value);
 }
 public void CanDeleteCurrentEntryWhenOperatorIsOnStack()
 {
     var engine = new Engine();
       engine.Digit(1);
       Assert.AreEqual(1, engine.Value);
       engine.Plus();
       Assert.AreEqual(1, engine.Value);
       engine.ClearEntry();
       Assert.AreEqual(0, engine.Value);
       engine.Digit(2);
       Assert.AreEqual(2, engine.Value);
       engine.Equals();
       Assert.AreEqual(3, engine.Value);
 }