In file included from main.cpp:16:0:
set.h: In member function 'Set Set::Union(const Set&) const':
set.h:48:16: error: could not convert 'un_v1' from 'std::vector<long int>' to 'Set'
         return un_v1;
                ^~~~~
set.h: In member function 'Set Set::Intersection(const Set&) const':
set.h:59:16: error: could not convert 'un_v3' from 'std::vector<long int>' to 'Set'
         return un_v3;
                ^~~~~
set.h: In member function 'Set Set::Difference(const Set&) const':
set.h:70:16: error: could not convert 'un_v1' from 'std::vector<long int>' to 'Set'
         return un_v1;
                ^~~~~
set.h: In member function 'Set Set::SymmetricDifference(const Set&) const':
set.h:83:16: error: could not convert 'un_v1' from 'std::vector<long int>' to 'Set'
         return un_v1;
                ^~~~~