[metapost] draw a single-stroke curve

[Qianqian Fang]

hi list

I want to produce a single-stroke curve in
metapost and import to fontforge, a vector
font editor.

I used the "draw" command and output to
a ps file, then import the ps file to fontforge.
I realize that, even I set the pen width to 0, the
imported ps graph is always an outline object:
every stroke is a thin closed contour.

what command I should use in metapost to
produce a ps file that can have single-stroke
one-directional curves?

thanks

Qianqian