Enforcing values of numbers

Is it possible to enforce that two numbers are within ranges relative to
each other.
i.e.  two elements x and y, x must be less the y.

I also have a problem where I need to have a list of Telephone number
segments that can have leading zeros that must be formatted as follows:

<Range>nnnn-mmmm</Range>

How can I enforce mmmm to be greater then nnnn.

Lastly, I need to have a list of these telno segments that occur only once
in a list
<List>123 456 789</List>
Would be legal.
<List>123 456 123</List>
would not be.

Any Help much appreciated.

Martin Roberts
xml designer, BTexaCT
01473 643775
martin.me.roberts@bt.com