|   | 1 |  initial version  | 
The following seems to work correctly.
var('a')
assume(a, 'complex')
b=a*a.conjugate()-a*a
simplify(b)
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.