Make an input invalid - a super quick question

Hello everyone.

I have a complicated set of rules that have to pass before my form is submitted.
To make it easier I am doing an initial constraint check which fires fine.
Then I am doing another check and if all the rules fail I want to make the input invalid...

is this possible?
I am hoping there is a flag or something to set an xforms input as invalid

e.g.

<xf:invalid ref="myNode/myItem" >

Hopefully you understand what I mean and you can help

thanks