[Groff] Bug in pic? And how to do this right?

[Greg 'groggy' Lehey]

I'm trying to draw a picture of a "volume" with pic.  Basically, it
should be an ellipse at the top, straight vertical sides, and the
front of an ellipse at the bottom.

Two things stop me from doing this:

1.  I can't do only part of an ellipse, or at least that's the way I
    read both the documentation and the code.

2.  I can't get an arc to start at the bottom of the left hand side.
    If I write the code the way it should be, it starts too far to the
    left.  If I gradually move the start to the right, it suddenly
    jumps past the reference point and leaves a gap.  This behaviour
    has been there for several years now.

I'm attaching the source code and the resultant PostScript output from
the following command:

   groff -p pic1.mm > pic1.ps

Questions (yes, the sequence is deliberate):

2.  This is a bug, right?  Any idea how to fix it?

1.  Am I correct in my assumption that I can draw only whole ellipses?
    Is there any other way to draw the correct partial ellipse?  It
    seems that it would be nice to have a feature in pic to draw
    arbitrary curves (specified by a formula), but I can't quite see
    how to do it.  If it's not there, and people think it's a good
    idea, I'm prepared to have a crack at it some time.  In that case,
    can anybody give me an idea how to go about it?  I've had a look
    in the code, and it doesn't look impossibly difficult or
    convoluted.

Greg
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pic1.mm
Description: Text document

pic1.ps
Description: PostScript document

pgp5auYRB04Fg.pgp
Description: PGP signature