🚀 go-pugleaf

Yeah, at the beginning of the file you find:

... subspline 1 1 n 3 sub { /i exch def 1 sub ... } for

Here subspline leaves nothing on the stack, the "/i exch def" consumes the cycle variable, so there is nothing on the stack to subtract 1 from.

Without comments it is some work to de-cypher what the program does.
Perhaps it would be easier to do something from scratch.

A (parametric) cubic spline goes through all of its given points with
no furher points in-between, a Bezier curve needs two more control
points. One can conjure up many schemes to automatically generate these control points, and some of them could look ok some of the time.
At other times they could generate overshoots and other undesirables.

Probably it would not take much more time to write a routine to generate parametric cubic splines than it would take to tinker with
Beziers to force them to do something they were not designed to do.

Miklos