[isabelle] Help on Nominal package

Hi all
I just start to use the new Nominal package. And I get the following
  error when I try to execute one of the examples provided in the package.
  *** Cyclic dependency of constants:
  ***   "nominal.perm"
  *** The error(s) above occurred in definition "perm_set_def":
  ***   "pi \<bullet> X == {pi \<bullet> a |a. a : X}"
  *** At command "defs" (line 26 of "/Users/lememta/Desktop/tesi di
  *** Error.
  *** At command "theory".*** Cyclic dependency of constants:
  Can somebody please give me a hint to solve this problem?
  Thanks in advance for the collaboration.

-Temesghen Kahsai

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