How to define a permutation with EBNF?
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I'm using EBNF to define a grammar.

But I'm stuck because I need to define a permutation : I have a set of values that can be combined, but they have to be used only once and I don't care about the order.

How to do it with EBNF ?

example:
values : a, b ,c

possible combinations : abc, acb, bac, bca, cab, cba
