[R] Solving systems of non-linear equations in R

[T Petersen]

I'm about to write my thesis in economics and will need to setup and 
solve a system of non-linear equations. At our university we usually use 
GAMS for this, and though GAMS is a fine program, it bugs me a that I 
won't be able to run my code after I finish my thesis without buying a 
license for the program(about $3.500 :-(( )

So I've looked around for NL-stuff for R, but I can't find anything. The 
closest thing appears to be optim(), but it doesn't seem to allow 
constraints(as in fn = constant) or equations systems. So, anyone knows 
if there is a method in R that you can use for this purpose?

regards