A strange grammer in C++ not operator==

I'm reading/compiling an open source package Trilinos. The source code could be found in Github. I found the following strange grammer that leads to compiling error in the intel compiler (windows) while gcc 4.9.x in Ubuntu works.

bool Slice::operator!=(const Slice & slice) const
{
  return (not operator==(slice));
} 

The error messages are:

Severity    Code    Description Project File    Line    Suppression State
Error       identifier "not" is undefined   domi    .\packages\domi\src\Domi_Slice.hpp  475 

Question: 1> What does the above grammer mean? 2> Is this C++ standard? Why does intel compiler not support the grammer?

Thanks very much.