Why doesn't std::sets of alglib::real_1d_array work in this case?

Question

I'm forking a C++ program using Alglib so that I can invert matrices and do matrix multiplications easily.

Right now I'm facing this error:

Invalid operands to binary expression ('const alglib::real_1d_array' and 'const alglib::real_1d_array')

So what caused this error?

 inline vecset getAdmittedCVectors(void)
 {
        return admittedCVectors;
 }

Here vecset means

typedef std::set<alglib::real_1d_array> vecset;

and admittedCVectors is a member of a class.

vecset admittedCVectors;

How shall I solve it? Using a variant of std::set that does not have this issue or overloading < in alglib::real_1d_array which is much harder?