View Compilation Error

0_0_22206757_16683.cpp: In function 'void seg_set(int, int, int, int, int, int)':
0_0_22206757_16683.cpp:51:13: error: 'mid' was not declared in this scope
     if(b <= mid) seg_set(a, b, v, lch(n), l, mid); //抛弃右段
             ^