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PyMapping_Keys is a macro -- it can't be exported, as there 
is no function of that name.  Its expansion is supplied by 
including Python.h.

PyMapping_Size is deliberately undocumented; it's an 
obsolete name for PyMapping_Length.  Note that the point 
isn't to export every conceivable entry point, it's to make the 
documented C API available.  A lot of stuff in the header files 
exists only for backward compatibility with obsolete spellings.