diff --git a/src/main/java/gov/usgs/earthquake/nshmp/data/XySequence.java b/src/main/java/gov/usgs/earthquake/nshmp/data/XySequence.java
index 9677a0a58c17a5d660c5929357ad9caae31386fc..bf08baafdd352e12ff72691f30d857f3b784976d 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/data/XySequence.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/data/XySequence.java
@@ -105,6 +105,7 @@ public interface XySequence extends Iterable<XyPoint> {
       Map<E, MutableXySequence> map,
       XySequence sequence) {
 
+    // TODO replace or deprecate/remove in favor of Map.merge
     if (map.containsKey(key)) {
       map.get(key).add(sequence);
     } else {