[R] nls segmented model with unknown joint points

[andy]

Hello,

I am interested in fitting a segmented model with unknown joint points
in nls and perhaps eventually in nlme.  I can fit this model in sas (see
below, joint points to be estimated are a41 and a41), but am unsure how
to specify this in the nlm function.  I would really appreciate any
suggestions or example code.  Thanks a lot. -andy  

proc nlin data=Stems.Trees;
 params  b41=-3 b42=1.5 b43=-1.5 b44=50 a41=0.75 a42=0.1;

 term1 = (b41*(x - 1) + b42*(x**2 -1));

 if (a41 - x) >= 0 then 
	term2 = (b43*(a41 - x)**2);
 else
	term2 = 0; 

 if (a42 - x) >=0 then
	term3 = (b44*(a42 - x)**2); 
 else 
	term3 = 0;

 model y = term1+term2+term3;
run;