Reverse engineer Connect Edges' Pinch

[thomthom]

I'm trying to implement a feature similar to 3dsMax's Connect Edges feature: http://docs.autodesk.com/3DSMAX/13/ENU/Autodesk%203ds%20Max%202011%20Help/index.html?url=./files/WSf742dab041063133-48601c13112a1cebbd0-7fae.htm,topicNumber=d0e116152

What I find troublesome is adding the pinching.

I have a method that calculates the ratios for each segment from 0.0 to 1.0 which I later use with Geom.linear_combination to get the 3d points.

I've been studying the behavior of the pinch property but I've not been able to work out an equation for both negative and positive pinch.
So far I've got one for positive and one for negative.

I'm wondering if it's possible to use one for both?

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