code: public String getPathNameRelativeTo(VirtualFile parent) throws IllegalArgumentException {
          final StringBuilder builder = new StringBuilder(160);
          getPathNameRelativeTo(parent, builder);
          return builder.toString();
      }
docs: Get the path name relative to a parent virtual file.  If the given virtual file is not a parent of
  this virtual file, then an {@code IllegalArgumentException} is thrown.
  
  @param parent the parent virtual file
  @return the relative path name as a string
  @throws IllegalArgumentException if the given virtual file is not a parent of this virtual file