MGCV:: boundary conditions in gam

Dear R-help,

I have a problem where I am using the mgcv package to in a situation where
I am fitting a gam model with a 1-D spline smoother model over a domain
[a,b] but then need to make predictions and extrapolate beyond b. Is there
anyway where I force the first derivative of the spline to be zero at
boundaries, so that I simply get a constant value outside the domain?

Best wishes,

Mark