namespace boost
{
    namespace qvm
    {
        //Only enabled if:
        //  is_quat<Q>::value
        
        template <class Q> -unspecified-return-type- S( Q & q );
        template <class Q> -unspecified-return-type- V( Q & q );
        template <class Q> -unspecified-return-type- X( Q & q );
        template <class Q> -unspecified-return-type- Y( Q & q );
        template <class Q> -unspecified-return-type- Z( Q & q );
    }
}

An expression of the form S(q) can be used to access the scalar component of the quaternion q. For example,

S(q) *= 42;

multiplies the scalar component of q by the scalar 42.

An expression of the form V(q) can be used to access the vector component of the quaternion q. For example,

V(q) *= 42

multiplies the vector component of q by the scalar 42.

The X, Y and Z elements of the vector component can also be accessed directly using X(q), Y(q) and Z(q).