code: 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;
      }
docs: Sets a file whose contents will be prepended to the JAR file's data.
  
  @param file the prefix file, or {@code null} for none.
  @return {@code this}