These function serve as intermediaries between R and the subdirectory source.
Details
The exported function name is different from the subdirectory function. C++ does not allow duplicate function names with the same parameters. To preserve the original function name necessitates the subdirectory function being placed behind a namespace, e.g. `A::calc_A_routine()`. Otherwise, the name of the function exported must be different, e.g. `calc_A_export()`.