[R] back-solver: any R thing like TK!Solver?

[Carl Witthoft]

Just thought I'd ask.  For those who've never seen TK!Solver, I strongly 
recommend taking a look.  So far as I can tell, it's the only product of 
its type available, retail or open source, for any platform.
What makes TK!Solver so cool is that it adaptively back-solves pretty 
much any unknown from any set of equations you give it.  (or vector full 
of unknowns, producing a vector of results, and so on)

I've been bummed for years 'cause the original package ran on Macs 
(inthe OS-4 days) but is now only available for WinBlows -- and is 
getting expensive.  So:  what sort of back-solver tools or packages are 
there in R, if any?

thanks
Carl