In file included from main.cpp:16:0:
set.h: In member function 'Set Set::Union(Set)':
set.h:60:12: error: could not convert 'res' from 'std::vector<long int>' to 'Set'
     return res;
            ^~~
set.h: In member function 'Set Set::Intersection(Set)':
set.h:68:12: error: could not convert 'res' from 'std::vector<long int>' to 'Set'
     return res;
            ^~~
set.h: In member function 'Set Set::Difference(Set)':
set.h:76:12: error: could not convert 'res' from 'std::vector<long int>' to 'Set'
     return res;
            ^~~
set.h: In member function 'Set Set::SymmetricDifference(Set)':
set.h:87:12: error: could not convert 'res' from 'std::vector<long int>' to 'Set'
     return res;
            ^~~