improve persistent class

lib/interfaces/Assignable.ts

@@ -1,34 +1,5 @@
 export interface Assignable {
   assign (a: Assignable): void;
   assign_to (a: Assignable): void;
-}
-
-export abstract class Persistent {
-  private data: Record<string, unknown> = {};
-  private properties: Record<string, string>;
-
-  public constructor() {
-    this.properties = this.define_properties();
-  }
-
-  public abstract define_properties(): Record<string, string>;
-
-  public assign (da: Assignable): void {
-    this.assign_object (da.get_data ());
-  }
-
-  public assign_to (da: Assignable): void {
-    da.assign (this);
-  }
-
-  public assign_object (obj: Record<string, unknown>): void {
-    for (const key of Object.keys (obj))
-      this.data[key] = obj[key];
-  }
-
-  public get_data (): Record<string, unknown> {
-    return this.data;
-  }
-
-  wip
+  to_object (): Record<string, unknown>;
 }