Confusion about the definition of the Language_Frame class

I am looking at the file language_frame.py, and the class Language_Frame. From the docstring it appears that the variable p_e is a list of extensional predicates. However, at line 21-22 you check if the predicate is extensional (i.e. not intensional) and if true return an error. This is confusing, what are those two lines supposed to check?
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