Al G wrote:
I thought in most of airlines' operation manual, a TCAS's manuever
instruction is a mendatory command that the crew must follow
immediately after its issurance.

This rings a bell with me too. It may have come about as a result of a
collision between a Tu154 and a B757 over Germany a few years back. The TCAS
said one thing, and the controller said another; it seems that although both
pilots obeyed the TCAS in the first instance, the Tu154 pilot changed his
mind and obeyed the controller instead. Thus both aircraft descended, and
the collision occurred - but it wouldn't have if TCAS had been obeyed in
both instances.

D.