Is it possible to restrict elements to a max occur in a “choice” block?

Question

I need to solve the following problem.

//pseudo algorithm

• you have four elements: elm1, elm2, elm3, elm4
• elm1 occurs 0-2 times
• elm2 o

Answer 1

It's not possible in xml-schema. You can use a choice in combination with something like schematron:

    
    
        an error message