This problem was solved by, among others, jhartman@scanner.frw.uva.nl.
Maybe he can tell you how it's done...

----\_\_\_--\_----\_------\_-------------------- P. Martijn van Leusen -------- 
     \_   \_ \_\_\_\_      \_        Dept of Pre- and Protohistoric Archaeology
      \_\_\_  \_ \_ \_      \_        University of Amsterdam, The Netherlands
       \_      \_    \_ \_\_ \_               martijn@scanner.frw.uva.nl
--------\_------\_----\_--\_--\_\_\_\_-----------------------------------------

