Apply suggestion to src/main/java/gov/usgs/earthquake/nshmp/data/Sequences.java

src/main/java/gov/usgs/earthquake/nshmp/data/Sequences.java

@@ -87,7 +87,7 @@ class Sequences {
    * @return xys
    */
   static XySequence combine(Collection<XySequence> xySequences) {
-    checkState(!xySequences.isEmpty());
+    checkState(areSimilar(xySequences));
     if (xySequences.size() == 1) {
       return getFirstSequence(xySequences);
     }