get enable_if out of the return type for nice error messages like "decltype(m) d = 3*s; // error : no viable conversion from quantity<[...], sequence<[...], 2>, [...]> to quantity<[...], sequence<[...], (unitscxx::si::units)0 aka 0>, [...]>"
get enable_if out of the return type
for nice error messages like "decltype(m) d = 3*s; // error : no viable conversion from quantity<[...], sequence<[...], 2>, [...]> to quantity<[...], sequence<[...], (unitscxx::si::units)0 aka 0>, [...]>"

zneak/units-cxx14