Purify complained about reading uninitialized memory in ast.c:752 of 
two bytes which corresponds to the type field.  Looking into this, line 
750 increments i without checking that there are in fact this many 
children.
If you add the line:
assert(i < NCH(n));
after line 750 you get an assertion failure when you run 
test_keywordonlyarg in the testsuite