[isabelle] Adding lemmas to fact collections


in Isabelle 2005 it was possible to successively add lemmas to a
collection of lemmas in a way like:
lemmas some_lemmas = lemma1 lemma2
lemmas some_lemmas = some_lemmas lemma3
lemmas some_lemmas = some_lemmas lemma4

In Isabelle 2008 this method yields the error message "Duplicate fact".
Is there a way to add new facts to an already declared fact collection?

Thank You in advance!


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