Fix checked_get

src/calc.rs

@@ -510,8 +510,9 @@ impl<'a> Calculator<'a> {
     }
 
     fn checked_get(&self, idx: usize) -> CalculatorResult<f64> {
-        self.stack
+        match self.stack.get(idx) {
-            .get(idx)
+            None => Err(CalculatorError::NotEnoughStackEntries),
-            .ok_or(Err(CalculatorError::NotEnoughStackEntries))
+            Some(r) => Ok(*r),
+        }
     }
 }