View Compilation Error

0_0_15345409_12726.cpp: In function 'long long int PowerMod(const long long int&, const long long int&, const long long int&)':
0_0_15345409_12726.cpp:16:41: error: 'exp_mod' was not declared in this scope
     long long tmp=exp_mod(base,exp/2,mod);  
                                         ^