Multi-Valued Dependencies

In Date's most recent edition (7th) of his book, "An Introduction to Database Systems," he defines a multi-valued dependency (MVD) as follows:

  Let R be a relvar, and let A, B, and C be subsets of the attributes   of R. Then we say that B is multi-dependent on A---in symbols,

  A ->-> B

  (read "A multi-determines B," or simply "A double arrow B")---if and   only if, in every possible legal value of R, the set of B values   matching a given (Avalue, C value) pair depends only on the A value   and is independent of the C value.

Fine. With this definition in place, he then goes on to analyze the course/teacher/text problem:

  COURSE       TEACHER         TEXT
  Physics      Green           Basic Mechanics
  Physics      Green           Principles of Optics
  Physics      Brown           Basic Mechanics
  Physics      Brown           Principles of Optics
  Math         Green           Basic Mechanics
  Math         Green           Vector Analysis
  Math         Green           Trigonometry

He then goes on to decompose this relvar into course/teachers and course/texts:

  COURSE       TEACHER
  Physics      Green
  Physics      Brown
  Math         Green

  COURSE       TEXT
  Physics      Basic Mechanics
  Physics      Principles of Optics
  Math         Basic Mechanics
  Math         Vector Analysis
  Math         Trigonometry

OK, now here is the issue I have: At this point, Date asserts that the course/teachers and course/texts relvars have no MVDs. I disagree. Consider the course/teachers relvar, and let A=COURSE, B=TEACHER, and C=nullset. Then A ->-> B.

Is this incorrect? If so, can someone please tell me why?

-- 
% Randy Yates                  % "And all that I can do
%% DIGITAL SOUND LABS          %   is say I'm sorry, 
%%% Digital Audio Sig. Proc.   %   that's the way it goes..."
%%%% <yates_at_ieee.org>          %'Getting To The Point', *Balance of Power*, ELO
http://207.87.184.178/index.htm