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--* Subject: [9] cannot test failure of open$InPort [inport.txt][0.35.0]

--@ Fixed  by:  PI   Fri Aug 5 17:51:26 EDT 1994 
--@ Tested by:  none 
--@ Summary:    No more a bug. With the new library you can import from InFile and use open and open? 

There is currently no way of checking whether an open from InPort was
successful. Trying to use an InPort after a bad open (e.g. file doesn't
exist) causes a segmentation fault, so we need a  valid?: InPort -> Boolean
function to test those.