he drop target.
For each type, <I>oplist</I> is a list
{<I>descops</I> <I>mod</I> ?<I>descops</I> <I>mod</I>? ...} describing operations and
modifier keys for these operations.
<I>descops</I> describe an operation. It can be a predefined operations (<B>copy</B>,
<B>move</B> or <B>link</B>) or a new user defined operation, of the form {<I>subop</I>
<B>baseop</B> ?<I>bitmap</I>?}.
<I>subop</I> is the name given to the sub operation, <I>baseop</I> is the name of the
base operation (<B>copy</B>, <B>move</B> or 