;;;Hi Cab I did a test with your lisp , an it give not the , say, included angle among two consecutive lines .

;;;This is so, because the first derivative [ FD ] give the tangent of a curve at that point,

;;or also it give the idea how the curve is growing or decaying at that point.

;;;When curve is a line , it give the tangent at that point that in this case is the same as the line angle.

;;;Also in this case , I do not know if ACAD take the value for the ending line or starting line at a given parameter.

;;;I think that if we take the the FD at ( - param 0.1) , and at (+ param 0.1) , it will give 2 angles , and from it,

;; we can get angle between the lines.Not so sure.

;;;I'm doing now some test .

;;;From it I knew that the FD at Param give the angle for the after the param