// define X to activate the xerr/xerr2 macros:
//      xerr(insertion) 
//          inserts the '<<' concatenated elements into std::cerr
//          preceded by the name of the source file, and ended by '\n'
//      xerr2(insertion, code) 
//          performs the insertion if X is defined, and (unconditionally)
//          executes the statement(s) in `code'. `code' must be valid
//          C(++) code. 
//  
#ifdef XERR
    #include <iostream>
    #define xerr(insertion)  std::cerr << __FILE__": " << insertion << '\n'
    #define xerr2(insertion, b) \
                    { std::cerr << __FILE__": " << insertion << '\n';  b;  }
#else
    #define xerr(insertion) 
    #define xerr2(insertion, b) b
#endif

