diff --git a/integer.cpp b/integer.cpp
index 0a8c544..d6db35d 100644
--- a/integer.cpp
+++ b/integer.cpp
@@ -128,7 +128,18 @@ public:
     }
     return lhs;
   }
-  friend Integer &abs_sub(Integer &lhs, Integer const &rhs) { return lhs; }
+  friend Integer &abs_sub(Integer &lhs, Integer const &rhs) {
+    assert(lhs > rhs);
+    auto t_size = rhs.true_size();
+    bool overflow = false;
+    for (std::size_t i = 0; i < t_size; i++) {
+      if (rhs.bytes[i] + (overflow ? 1 : 0) > lhs.bytes[i]) {
+        overflow = true;
+      }
+      lhs.bytes[i] -= rhs.bytes[i] + (overflow ? 1 : 0);
+    }
+    return lhs;
+  }
   friend Integer &operator+=(Integer &lhs, Integer const &rhs) {
     // TODO: negative numbers
     if (lhs.isNegative == rhs.isNegative) {