View Compilation Error

0_0_27153251_9524.cpp: In member function 'bool Point3::operator==(Point3)':
0_0_27153251_9524.cpp:30:49: error: 'sgn' was not declared in this scope
     bool operator == (Point3 B){return sgn(x-B.x)==0 && sgn(y-B.y)==0 && sgn(z-B.z)==0;}
                                                 ^