def pop
      # NOTE: can't use popstr because the data might be binary
      ptr = ffi_delegate.pop
      return nil if ptr.null?
      Frame.from_ffi_delegate(ptr).to_s
    end
def pop
      # NOTE: can't use popstr because the data might be binary
      ptr = ffi_delegate.pop
      return nil if ptr.null?
      Frame.from_ffi_delegate(ptr).to_s
    end
Removes first part from message and returns it as a string.
 @return [String, nil] first part, if any, or nil