[metapost] envelope

[Akira Kakuto]

> A classical  example is
>     path P;
>     pen A;
>     P:=(0,0){up}..(100,0){down}..cycle;
>     A:=pensquare scaled 10;
>     draw P withpen A;

In the latest MetaPost in the trunk,

(1) P:=(0,0){up}..(100,0){down}..cycle;
(2) P:=(0,0){down}..(100,0){up}..cycle;
(3) P:=(50,-50){left}..(50,50){right}..cycle;
are NG.
But
(4) P:=(50,-50){right}..(50,50){left}..cycle;
seems to be OK.
I don't know why.

Best,
Akira