DTD required elements ordering

Question

I want to have list of required elements in any order defined in dtd, but have no idea how I could do this.

For example, I have following definition:

Answer 1

This is tricky when you need exactly one of each child elements. This is the only way I can think of and it's not very pretty. It works though...

Basically I'm specifying every possible combination of exactly one child1, child2, and child3 elements.

Here's an example instance. child1, child2, and child3 can be in any order, but they each must occur exactly one time.

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