progress with versioning lib

lib/structures/struct.js

@@ -28,3 +28,13 @@ export const makeGetters = fieldNames => {
     }[getterName];
   })
 };
+
+export const makeStructCompareFn = compares => {
+  return a => b => {
+    for (const {l: fieldName, r: getCompareFn} of compares) {
+      const cmp = getCompareFn()(a[fieldName])(b[fieldName]);
+      if (cmp !== 0) return cmp;
+    }
+    return 0;
+  }
+}