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(B4) if you have a stack of generators where each is in 'yield from' from
 the next one, and you call '.next()' on the outermost, then it enters
 and leaves all intermediate frames. This is costly but may be
 required to get the sys.settrace()/setprofile() hooks called.
 However, if you call '.throw()' or '.close()' instead, then it uses a
 much more efficient way to go from the outermost to the innermost
 frame---as a result, the enter/leave of the intermediate frames is not
 invoked. This can confuse coverage tools and profilers. For example,
 in a stack ``f1()->f2()->f3()``, vmprof would show f3() as usually
 called via f2() from f1() but occasionally called directly from f1().