size_t index ( T const& x,size_t const& l, size_t const& r) const{


    if(l > size()||r>size())
        throw("Menge::index(): index out of range.");

    //cut the interval
    size_t m = (l+r)/2;

    // x was found
    if( x == (*this)[m])
        return m;

    // x can't be found
    if( l==m)
        return NPOS;

    //rekursive part
    if( x < (*this)[m])
        return index(l,m,x);

    return index(m+1,r,x);

}

bool contains ( T const& elem ) const{
    return index(elem, 0, size()-1)!=NPOS;
}

size_t index (T const& x, size_t const& l=0, size_t const& r=size()-1)const;

size_t index (T const& x, size_t const& l=0, size_t const& r=this->size()-1)const;