progress and some refactoring

util/defaultmap.js

@@ -0,0 +1,18 @@
+
+export class DefaultMap {
+  constructor(defaultValue, ...rest) {
+    this.defaultValue = defaultValue;
+    this.m = new Map(rest);
+  }
+  getdefault(key, addToMapping = false) {
+    return this.m.get(key) || (() => {
+      const val = this.defaultValue(key);
+      if (addToMapping)
+        this.m.set(key, val);
+      return val;
+    })();
+  }
+  entries() {
+    return this.m.entries();
+  }
+}