Difficult logical relationship in complex type

I'm having difficulties defining a logical relationship within a complex type.  The complex type contains 4 sub-elements.  Let's call them A, B, C, D.  The logical relationship is this:


1.       One, or more of elements B, C, or D are required.

2.       There may be zero or more instances of element A.

3.       Element A may never be the only element.  In other words, there may only be an instance of A, if there is also at least one instance of B, C, or D.

If I was just dealing with B, C, and D, I could handle that.  However, adding the requirements for A has thrown me.

Thanks for any responses, in advance.