View Compilation Error

0_0_20293411_14439.cpp: In function 'void graham(int)':
0_0_20293411_14439.cpp:52:34: error: 'x_multi' was not declared in this scope
   if(sgn(x_multi(p[i],p[i-1],p[0]))) p[tot++]=p[i-1];
                                  ^
0_0_20293411_14439.cpp:59:53: error: 'x_multi' was not declared in this scope
   while(top>=1&&sgn(x_multi(p[i],sta[top],sta[top-1]))>=0)
                                                     ^