View Compilation Error

0_0_35191038_22854.cpp: In function 'bool G2::isright(G2::line, G2::point)':
0_0_35191038_22854.cpp:70:64: error: 'cross' was not declared in this scope
  inline bool isright(line a,point b){return cross(a.t-a.s,b-a.s)<0;}
                                                                ^
0_0_35191038_22854.cpp: In function 'G2::point G2::intersection(G2::line, G2::line)':
0_0_35191038_22854.cpp:74:46: error: 'cross' was not declared in this scope
   return a.s+(a.t-a.s)*(cross(a.s-b.s,b.t-b.s)/cross(b.t-b.s,a.t-a.s));
                                              ^