equivalent of NOT NULL for element values?

Hello, we're using Schema to validate a SOAP feed - I'd like to  
require that certain elements be supplied without "null" (for lack of  
a better word) values:

<some_element></some_element>

Ideally this would be illegal according to the Schema regardless of  
the type of data that the element contained.

If this is not possible, or rather if it's acheived by restrictions  
in simpleType definitions, how can I impose a non-null value for an  
integer type which may be unbounded positive or negative?

thank you,
-lev