diff --git a/src/lib.rs b/src/lib.rs
index 698e86f..06d39af 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,5 @@
 use std::fmt;
-use std::ops::Add;
+use std::ops::{Add, AddAssign};
 
 /// This enum represents the sum of a sequence of numbers that may be integers or floating point.
 /// Integer is the default. When a floating point number is added to the sum, the type is converted
@@ -24,6 +24,13 @@ impl Add for Sum {
     }
 }
 
+impl AddAssign for Sum {
+    /// Adds two Sums. If either is a Float, the result will be a Float.
+    fn add_assign(&mut self, other: Self) {
+        *self = *self + other;
+    }
+}
+
 impl fmt::Display for Sum {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
@@ -51,6 +58,10 @@ mod tests {
         let a = Sum::Integer(1);
         let b = Sum::Integer(2);
         assert_eq!(a + b, Sum::Integer(3));
+
+        let mut c = a;
+        c += b;
+        assert_eq!(c, Sum::Integer(3));
     }
 
     #[test]
diff --git a/src/main.rs b/src/main.rs
index a7cfea3..514ee76 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -79,7 +79,7 @@ fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
                     }
                 }
             };
-            sum = sum + n;
+            sum += n;
             log::debug!("{i}: col={col:?}, n={n:?}, sum={sum:?}");
         }
     }