diff --git a/src/bigint.rs b/src/bigint.rs
index e5ebd37..336ec7c 100644
--- a/src/bigint.rs
+++ b/src/bigint.rs
@@ -2,11 +2,12 @@
 //! \[WIP\] Arbitrarily large integers
 use crate::num::Sign::*;
 use crate::num::*;
+use core::convert::TryInto;
+use core::mem::replace;
 use core::ops::{
     Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Not, Rem, RemAssign, Sub, SubAssign,
 };
 use core::usize;
-use std::convert::TryInto;
 
 #[derive(Clone, Debug, PartialEq)]
 pub struct BigInt {
@@ -242,7 +243,7 @@ impl Mul for BigInt {
             }
         }
 
-        out.trim_zeros();
+        out.shrink_to_fit();
 
         out
     }
@@ -266,31 +267,36 @@ impl Rem for BigInt {
 
 impl AddAssign for BigInt {
     fn add_assign(&mut self, rhs: Self) {
-        *self = self.clone() + rhs;
+        let this = replace(self, BigInt::new());
+        *self = this + rhs;
     }
 }
 
 impl SubAssign for BigInt {
     fn sub_assign(&mut self, rhs: Self) {
-        *self = self.clone() - rhs;
+        let this = replace(self, BigInt::new());
+        *self = this - rhs;
     }
 }
 
 impl MulAssign for BigInt {
     fn mul_assign(&mut self, rhs: Self) {
-        *self = self.clone() * rhs;
+        let this = replace(self, BigInt::new());
+        *self = this * rhs;
     }
 }
 
 impl DivAssign for BigInt {
     fn div_assign(&mut self, rhs: Self) {
-        *self = self.clone() / rhs;
+        let this = replace(self, BigInt::new());
+        *self = this / rhs;
     }
 }
 
 impl RemAssign for BigInt {
     fn rem_assign(&mut self, rhs: Self) {
-        *self = self.clone() % rhs;
+        let this = replace(self, BigInt::new());
+        *self = this % rhs;
     }
 }
 
@@ -445,6 +451,21 @@ mod tests {
         assert_eq!(sum.inner[1], 1usize);
     }
 
+    #[test]
+    fn test_add_assign() {
+        let mut a = BigInt::from(core::usize::MAX);
+        let b = BigInt::from(core::usize::MAX);
+
+        a += b;
+
+        assert_eq!(
+            a.inner[0],
+            core::usize::MAX - 1,
+            "least significant place should be MAX - 1"
+        );
+        assert_eq!(a.inner[1], 1usize, "most significant place should be 1");
+    }
+
     #[test]
     fn test_sub() {
         let a = BigInt::from(core::usize::MAX);
@@ -484,6 +505,69 @@ mod tests {
         assert_eq!(a.inner, vec![core::usize::MAX, core::usize::MAX]);
     }
 
+    #[test]
+    fn test_mul() {
+        let a = BigInt::from(65536);
+        let b = BigInt::from(4);
+
+        let product = a * b;
+        assert_eq!(product.inner[0], 65536usize * 4);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_mul_overflow() {
+        let a = BigInt::from(core::usize::MAX);
+        let b = BigInt::from(5);
+
+        let product = a * b;
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_mul_assign_overflow() {
+        let mut a = BigInt::from(core::usize::MAX);
+        let b = BigInt::from(5);
+
+        a *= b;
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_div() {
+        let a = BigInt::from(128);
+        let b = BigInt::from(32);
+
+        let quotient = a / b;
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_div_assign() {
+        let mut a = BigInt::from(128);
+        let b = BigInt::from(32);
+
+        a /= b;
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_rem() {
+        let a = BigInt::from(5);
+        let b = BigInt::from(2);
+
+        let rem = a % b;
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_rem_assign() {
+        let mut a = BigInt::from(5);
+        let b = BigInt::from(2);
+
+        a %= b;
+    }
+
     #[test]
     fn test_zeros() {
         let a = BigInt::new();
@@ -506,21 +590,6 @@ mod tests {
         assert_eq!(b.inner[0], core::usize::MAX);
     }
 
-    #[test]
-    fn test_add_assign() {
-        let mut a = BigInt::from(core::usize::MAX);
-        let b = BigInt::from(core::usize::MAX);
-
-        a += b;
-
-        assert_eq!(
-            a.inner[0],
-            core::usize::MAX - 1,
-            "least significant place should be MAX - 1"
-        );
-        assert_eq!(a.inner[1], 1usize, "most significant place should be 1");
-    }
-
     #[test]
     fn test_from() {
         // Signed numbers
@@ -529,4 +598,16 @@ mod tests {
         // Larger than usize
         assert_eq!(BigInt::from(45u128), BigInt::from(45usize));
     }
+
+    #[test]
+    #[should_panic]
+    fn test_from_large_unsigned() {
+        BigInt::from(core::u128::MAX);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_from_large_signed() {
+        BigInt::from(128i128);
+    }
 }