Does anybody know how to implement a continuous numbering of chapters in
documents with several parts?

More specifically: how can I achieve the following numbering scheme (with
unprefixed chapter numbers):
I First part
1 Chapter one
2 Chapter two
II Second part
3 Chapter three


I tried (unsuccessfully) the following:

\setuphead[part][placehead=yes]
\starttext
\part{First part}
\chapter{Chapter one}
\chapter{Chapter two}
\part{Second part}
\chapter{Chapter three}
\stoptext

The here result is something like:
First part
1.1 Chapter one
1.2 Chapter two
Second part
2.1 Chapter three


I tried to get rid of the chapter prefixes by putting
\setuphead[chapter][prefix=no]
but that had no effect whatsoever.

Any ideas?

Julian