template[F]
function sin(x:F): F;

template[F]
function cos(x:F): F;

template[F]
function sincos(x:F): Pair[F,F]
where sin[F], cos[F]
{
    return {sin(x), cos(x)};
}

instance sin[double](x:double):double{
    // ...
}

instance cos[double](x:double):double{
    // ...
}

function main():int{
    // looks up sin[double] and cos[double]
    let sc = sincos(1.0);
}