[R] solving for a transition point in a piecewise nonlinear model

[Robert Musk]

I want to fit a piecewise nonlinear model in which the transition point is 
an estimated parameter.

Something like:

F(x)=F1(x) when x>alpha,
F(x)=F2(x) when x<=alpha.

How can I solve for alpha within the nls call?

Thanks,
Rob