[ccp4bb] How to subtract one electron density map from another

[Qing Xie]

Hi,
I'm trying to get the difference map by subtracting the native electron 
density map from the complex electron density map. MAPMASK has a function 
of ADD/MULT, but I don't know how to use it?
Any other ways to attack this problem in real space?
Thanks in advance,

Qing