implicit fitting algorithm

hi agan igor all

another attribution question. the fitting with user generated functions, fitfunc has the option to fit data without specifying any independent variables. it is very handy for closed curve functions in 2d  where there are multiple y values for every x, and cases where the function can be easy arranged to  have a zero, but the variables can't be conveniently written to isolate a dependent.. there was a discussion in the late teens on this forum about fitting an ellipse, where i became aware of this handy capability

besides having the ability to do this routinely, a wonderful gift from our heroes at wm, it works extremely well given a decent first guess. astonishingly well, often in a single iteration. there is not a lot of documentation on this and no reference to the source algorithm. is there a citation or is this another wavemetrics innovation? nick