[NTG-context] keeping output

This question must have been asked before, but I cannot find it. Running texexec a number of messages flash by: error in metapost run : syllabus-s-mpgraph.mp:397 Alas, al the intermediate results disappear, especially the mpgraph files. And of the metapost logs also one only survives. How does one preserve all these for debugging purposes? Hans van der Meer