[Users] Add a lapse condition

Dear all

Is there a way to add different gauge conditions for the lapse? I'm trying to add a gauge shock avoid lapse for the Bona-Masso family of slicing conditions

dot alpha = -alpha^2 f(alpha) K

with f(alpha)=1+constant/alpha^2

I don't know if I should add a thorn that complements the McLachlan thorn, or what is the correct procedure to implement the gauge condition? Furthermore, this kind of gauge condition allows to the lapse become negative which I know is a serious drawback when the lapse collapses to zero, however I want to do some numerical experiments with this kind of lapse.

Any help is very welcome. Thanks Erik