increase numerical tolerance

Hi,

I'm drawing a picture where I want to cut ellipses inscribed into a circle before and after they meet the
circle. Mostly this works but some ellipses are not cut presumably because metapost thinks that they do
not intersect. Is there any way to increase the numerical tolerance for two paths to intersect? Since I
know for sure that the paths intersect, I could then write an operator that increases the tolerance until
the paths finally intersect. Thanks in advance!

Best regards,
Stefan Witzel
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