question for metapost experts

["Schmitz Thomas A." <thomas.schmitz@uni-bonn.de>]

Hi all,

this one is for all the metapos/metafun experts out there (Alan, are you there?). For a counter in one of my slide backgrounds, I have to calculate an intersectionpoint, given in percentage of the difference between current page and last page. I have always had some difficulties on the first run, when the value of the last page isn’t known yet and hence not written to the tuc file, but the error 

> ! The paths don't intersect.
> <to be read again>


used to disappear in subsequent runs. Now, ConTeXt doesn’t compile the file at all and stops on this error. There is an error message I haven’t seen before:

> intersectionpoint->...The paths don't intersect");
> origin.else:0.5[point.x_.o...
> <to be read again>
> ;
> <*> ... ;z13 = (z11 -- z12) intersectionpoint qw ;
> draw z13 withcolor (0.1,0....
> This error message was generated by an `errmessage'
> command, so I can't give any explicit help.
> Pretend that you're Miss Marple: Examine all clues,
> and deduce the truth by inspired guesses.

Is there a way to make this conditional, such as (pseudocode)

if q intersects p:
 z = q intersectionpoint p ;
else:
 z = (0,0) ;
fi ; 

I hope that this would allow ConTeXt to proceed and calculate the point correctly on the second run.

Thanks, and all best

Thomas
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