  Given a physical quantum system described by a Hilbert H, for any bounded
quantum observable (a bounded self-adjoint operator) T it is possible to define
several ''hidden observable'' functions f:H->R associated to T and for any
quantum mixed state (a density matrix) D it is possible to define several
''hidden mixed states'' (probability measures) m on H associated to D in such a
way that the following equality is verified:
  Trace[ b(T). D] =integral[b(f(psi)).dm(psi) whatever is the continuous
function b:R->R. This formula gives a general way to express any expectation
value computable in a quantum theory as a classical statistical mean value.
