general inverse solver?

Just wondering if there's an R package which does tricks similar to what 
TK!Solver does.

TK!Solver, for those not lucky enough to have found it, basically allows 
one to define a bunch of equations, assign values to an arbitrary subset 
of the variables, from which it calculates (either directly when 
possible or back-solving when not) the values of the other variable(s).


thanks
Carl