Associates the specified value with the specified key in the specified
multimap. If the multimap previously contained a mapping for
the key, the existing value associated for the given key will be return and the
multimap is unchanged. (An multimap <tt>mm</tt> is said
to contain a mapping for a key <tt>k</tt> if and only if
m.containsKey(k) would return <tt>true</tt>.)
Associates the specified value with the specified key in the specified multimap. If the multimap previously contained a mapping for the key, the existing value associated for the given key will be return and the multimap is unchanged. (An multimap <tt>mm</tt> is said to contain a mapping for a key <tt>k</tt> if and only if m.containsKey(k) would return <tt>true</tt>.)