[pstricks] msave/mrestore

[vin gui]

I am wondering how to draw an elliptic arc, but with arrows.
I saw on the archives several answers, but not handling arrows.

So I tried the following thing:

% this works, but the linewidth is not constant (it is stretched by the 
rescaling).
% Also, if you add arrows they look very bad (they are probably also 
stretched)
\pscustom[linewidth=5mm]{
\scale{0.05 1} \psarc{<->}(0,-5){5}{45}{180}
}

I don't exactly understand what msave and mrestore should do but, 
(hoping that I would get the correct thing),
I tried this an this gave a postscript error.
Is this normal ? Does it have a chance to give a correct result ?

%this gives a postscript error
\pscustom[linewidth=5mm]{
\msave \scale{0.05 1} \psarc{<->}(10,-5){5}{45}{180} \mrestore
}

By the way, the following minimal example also gives a postscript error:

%this also gives a postscript error !
\pscustom{\msave  \mrestore }


Many thanks,

Vincent.