netgen export mesh got error: no member named 'WriteSTLFormat' in namespace 'netgen'

[AlexLuya]

I haved tried to export netgen meshing result as stl like this

netgen::WriteSTLFormat(*occ_mesh, "test_occ.stl");

and compiler emits error:

no member named 'WriteSTLFormat' in namespace 'netgen'

but WriteSTLFormat do existed in namespace of netgen and if trying to include:

#include </opt/netgen/include/interface/writeuser.hpp>

where WriteSTLFormat sits,got more errors like:

/opt/netgen/include/interface/writeuser.hpp:14:23: error: unknown type name 'Mesh'
                const Mesh & mesh,
                      ^
/opt/netgen/include/interface/writeuser.hpp:15:23: error: unknown type name 'NetgenGeometry'
                const NetgenGeometry & geom,
                      ^
/opt/netgen/include/interface/writeuser.hpp:23:16: error: unknown type name 'Mesh'
void ReadFile (Mesh & mesh,
               ^
/opt/netgen/include/interface/writeuser.hpp:24:22: error: unknown type name 'string'; did you mean 'std::string'?
               const string & filename);
                     ^~~~~~
                     std::string
...

Would you please tell me:

How to export netgen meshing result as stl correctly?

Thanks!!!