Operand.hpp:70:64: error: passing ‘const std::map<char, double (Operand<double>::*)(const double&, const double&)const, std::less<char>, std::allocator<std::pair<const char, double (Operand<double>::*)(const double&, const double&)const> > >’ as ‘this’ argument of ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = char, _Tp = double (Operand<double>::*)(const double&, const double&)const, _Compare = std::less<char>, _Alloc = std::allocator<std::pair<const char, double (Operand<double>::*)(const double&, const double&)const> >, std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = double (Operand<double>::*)(const double&, const double&)const, std::map<_Key, _Tp, _Compare, _Alloc>::key_type = char]’ discards qualifiers [-fpermissive]