View Compilation Error

0_0_36553595_32184.cpp: In function 'll qpow(ll, ll)':
0_0_36553595_32184.cpp:18:47: error: 'assert' was not declared in this scope
 ll qpow(ll a,ll b){ll ans=1;a%=mod;assert(b>=0);for(;b;b>>=1){if(b&1)ans=ans*a%mod;a=a*a%mod;}return ans;}
                                               ^