View Compilation Error

0_0_37644032_7551.cpp: In function 'void addL(long long int, long long int, long long int, long long int)':
0_0_37644032_7551.cpp:24:28: error: assignment of read-only variable 'mod'
  ((cL[l+1]+=(k-a)%mod)%=mod+=mod)%=mod;
                            ^
0_0_37644032_7551.cpp: In function 'void addR(long long int, long long int, long long int, long long int)':
0_0_37644032_7551.cpp:31:28: error: assignment of read-only variable 'mod'
  ((cR[l+1]+=(k-a)%mod)%=mod+=mod)%=mod;
                            ^