How to write own Evaluator

I am using glMap2f method as…
glMap2f(GL_MAP2_VERTEX_3, 0.0f,1.0f,3,nUOrder,0.0f,1.0f,3*nUOrder,nVOrder,fMeshCtls);
but it gives error GL_INVALID_VALUE…
here values of both nUOrder and nVOrder are 50.
glIntegerv(GL_MAX_EVAL_ORDER,&Result);
this method gives Result=30…
I read somewhere on internet that i want to write my own Evaluator but didn’t get any example…
So anybody can tell mi how to write own Evaluator…?
or how to solve this problem…?