code: func (m *Mrb) ConstDefined(name string, scope Value) bool {
  	cs := C.CString(name)
  	defer C.free(unsafe.Pointer(cs))
  
  	scopeV := scope.MrbValue(m).value
  	b := C.mrb_const_defined(
  		m.state, scopeV, C.mrb_intern_cstr(m.state, cs))
  	return C.ushort(b) != 0
  }
docs: // ConstDefined checks if the given constant is defined in the scope.
  //
  // This should be used, for example, before a call to Class, because a
  // failure in Class will crash your program (by design). You can retrieve
  // the Value of a Class by calling Value().