Breakpoint refinement fit

Breakpoint is a kind of refinement fit used to slowly and smoothly bend a curve. It's commonly used in table generation and for making small modifications. So, Christine would like it in the curve fitter too. Here are my explanations for the notes:

1. In the notes, ignore everything above the line. Those are notes for the eSeries we rewrote better elsewhere.
2. Ignore the --fit_type birch3 line.
3. She's calling it HPbreak because this is explicitly the breakpoint used for high density areas. There's a different breakpoint formula used on the other side of rho0.
4. rhob (not rho0!) is the density point from which the breakpoint comes into effect. rhob should be automatically calculated.
5. n may be either 2 or 3, nothing else. (3 has a stronger effect than 2.) Christine recommends trying to fit 2 functions, one with 2 one with 3, then figuring out which is closer.
6. Dang, I don't know why she has both rho < rhob and rho >= rhob on this page.
7. Notice the actual breakpoint function is defiend by the variable 'u' = (rho/rhob) - 1
   pbreak.pdf