m> was bound previously, that
    information is lost. Thus you cannot bind one user-defined
    attribute variable to multiple indices, but you can bind
    multiple user-defined attribute variables to the same
    index.</p><p>Applications are allowed to bind more than one
    user-defined attribute variable to the same generic vertex
    attribute index. This is called <span class="emphasis"><em>aliasing</em></span>,
    and it is allowed only if just one of the aliased attributes is
    active in the ex