code: @CheckForNull
      public String getPath() {
          Source parent = getParent();
          if (parent != null)
              return parent.getPath();
          return null;
      }
docs: Returns the File currently being lexed.
  
  If this Source is not a {@link FileLexerSource}, then
  it will ask the parent Source, and so forth recursively.
  If no Source on the stack is a FileLexerSource, returns null.