ambiguity in the tech file and therefore also in the .ext file.
This is easy to resolve and it was only necessary to change the
type after finding the tile belonging to the device. There are
pathological cases where this might still go wrong, like defining
a "short" device name for metal resistors on any plane; then,
if a "short" device overlaps something on another plane, it might
be given the wrong type. Avoiding all ambiguity would require
adding an extra item to "device" lines in the .ext file output;
the short plane name would be most efficient.
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