Added an argument selector
Added an argument selector

It returns the "correct" argument out of two.

rettichschnidi/sqlpp11,volka/sqlpp11,rbock/sqlpp-concepts-experiments,sergeken/sqlpp11,rbock/sqlpp-concepts-experiments,rbock/sqlpp-concepts-experiments,sergeken/sqlpp11,rettichschnidi/sqlpp11,happybits/sqlpp11,volka/sqlpp11,happybits/sqlpp11,rbock/sqlpp11,coldzhang/sqlpp11,rbock/sqlpp11,rettichschnidi/sqlpp11,happybits/sqlpp11,happybits/sqlpp11,coldzhang/sqlpp11,rbock/sqlpp11,rbock/sqlpp11,rettichschnidi/sqlpp11,sergeken/sqlpp11,volka/sqlpp11,volka/sqlpp11,sergeken/sqlpp11,coldzhang/sqlpp11