[Maxima-bugs] [ maxima-Bugs-2010843 ] diff of Taylor poly

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Bugs item #2010843, was opened at 2008-07-04 12:52
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Category: None
Group: Includes proposed fix
Status: Open
Resolution: None
Priority: 6
Private: No
Submitted By: Barton Willis (willisbl)
Assigned to: Nobody/Anonymous (nobody)
Summary: diff of Taylor poly 

Initial Comment:
Stanislav Maslovski discovered this bug and reported to the mailing list:

>============= 8< ===============
>(%i1) display2d:false;
>
>(%o1) false
>(%i2) sqrt((f - z^2/4/f)^2 + (d-z)^2) + sqrt((f - z^2/4/f)^2 + (zp-z)^2);
>
>(%o2) sqrt((zp-z)^2+(f-z^2/(4*f))^2)+sqrt((f-z^2/(4*f))^2+(d-z)^2)
>(%i3) taylor(%, [z, zp, d], 0, 4);
>
>(%o3) 2*f+(z^2+(-2*zp-2*d)*z+zp^2+d^2)/(2*f)
>         +((2*zp+2*d)*z^3+(-5*zp^2-5*d^2)*z^2+(4*zp^3+4*d^3)*z-zp^4-d^4)
>                /(8*f^3)
>(%i4) diff(%, z);
>
>Maxima encountered a Lisp error:
>
> Error in PROGN [or a callee]: Bad plist (($ZP ((4 . 1)) 0

A possible fix is to change old-get to

(defun old-get (plist tag)
 (oldget plist tag))

Currently old-get is a macro:  `(getf (cdr ,plist) ,tag)); here I changed it to a function (maybe it
needs to b a macro, I don't know). After rebuilding, the bug is gone and the testsuite reports no errors.

There are other bug reports related to a "bad plist"
Maybe these bugs are related.

Barton
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