diff --git a/src/lib.rs b/src/lib.rs
index 1746f69..c9f1013 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -339,6 +339,8 @@ mod test {
     #[test]
     fn check_array_ref_5() {
         fn f(data: Vec<u8>, offset: usize) -> quickcheck::TestResult {
+            // Compute the following, with correct results even if the sum would overflow:
+            //   if data.len() < offset + 5
             if data.len() < 5 || data.len() - 5 < offset {
                 return quickcheck::TestResult::discard();
             }
@@ -351,6 +353,8 @@ mod test {
     #[test]
     fn check_array_ref_out_of_bounds_5() {
         fn f(data: Vec<u8>, offset: usize) -> quickcheck::TestResult {
+            // Compute the following, with correct results even if the sum would overflow:
+            //   if data.len() >= offset + 5
             if data.len() >= 5 && data.len() - 5 >= offset {
                 return quickcheck::TestResult::discard();
             }
@@ -364,6 +368,8 @@ mod test {
     #[test]
     fn check_array_mut_ref_7() {
         fn f(mut data: Vec<u8>, offset: usize) -> quickcheck::TestResult {
+            // Compute the following, with correct results even if the sum would overflow:
+            //   if data.len() < offset + 7
             if data.len() < 7 || data.len() - 7 < offset {
                 return quickcheck::TestResult::discard();
             }
@@ -377,6 +383,8 @@ mod test {
     #[test]
     fn check_array_mut_ref_out_of_bounds_32() {
         fn f(mut data: Vec<u8>, offset: usize) -> quickcheck::TestResult {
+            // Compute the following, with correct results even if the sum would overflow:
+            //   if data.len() >= offset + 32
             if data.len() >= 32 && data.len() - 32 >= offset {
                 return quickcheck::TestResult::discard();
             }