boolean Filter - only hardcoded "is true"

Hello,

im trying to filter a resultset returned by a webservice.

each resultset-entry has a boolean "done"-flag, i want to filter those with the expression "done is false".

however if selecting this boolean-field, VC does not allow to enter an arbritrary expression, instead it just hardcodes an "is true" expression.....

any hints? some way to negate this?

thanks,

kai