This was triggered by a visual studio compile warning:

  cdef_test.cc(128):
  warning C4804: '>>': unsafe use of type 'bool' in operation

However the code is rather hard to parse for humans too: when I first
looked, I thought this was something to do with C++ templating...

The new version is equivalent but defines max_pos in an outer
loop (and a smaller indent).

Change-Id: I0c5cabeee44d0839a7956a4ab1cf4ec5abfcc9ee