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