如何根据兄弟元素的数量更改属性的必需/可选项？

I have a prototype XML schema - see end of post for an extract. What I'd like to do is to make the "row" and "column" attributes optional if there is only 1 "input" element, but required if there are multiple "input" elements. How do I do this in XSD syntax?I have a prototype XML schema - see end of post