docs: public Jar setJarPrefix(Path file) {
          verifyNotSealed();
          if (jos != null)
              throw new IllegalStateException("Really executable cannot be set after entries are added.");
          if (file != null && jarPrefixStr != null)
              throw new IllegalStateException("A prefix has already been set (" + jarPrefixStr + ")");
          this.jarPrefixFile = file;
          return this;
      }