XSD constraint based on tag value

[Bala Gangadhar]

Hi,
I am defining an xsd.
I have a xml tag <CCtype> which accepts one of the enumarated values.
And based on the value of <CCtype> i have to impose a constraint on another tag as mandatory or optional
for example if <CCtype> is Master card then <ccid> must be present and if <CCtype> is VISA then <ccid> is optional.

Can somebody suggest me how to implement this constraint..

Regards,
Bala.