addition: reciprocal, abs, is zero and is int

src/lib.rs

@@ -33,24 +33,28 @@ impl Fraction {
         let mut new = Fraction { num, den };
 
         new.reduce();
+        new.correct_sign();
 
         Ok(new)
     }
 
     pub fn reciprocal(&self) -> Result<Self, FractionError> {
-        todo!()
+        Fraction::new(self.den, self.num)
     }
 
     pub fn abs(&self) -> Self {
-        todo!()
+        Fraction {
+            num: self.num.abs(),
+            den: self.den,
+        }
     }
 
     pub fn is_zero(&self) -> bool {
-        todo!()
+        self.num == 0
     }
 
     pub fn is_integer(&self) -> bool {
-        todo!()
+        self.den == 1
     }
 
     fn gcd(a: i64, b: i64) -> i64 {