  We revisit the well known Bohr-Sommerfeld quantization rule (BS) for a 1-D
Pseudo-differential self-adjoint Hamiltonian within the algebraic and
microlocal framework of Helffer and Sj\"ostrand; BS holds precisely when the
Gram matrix consisting of scalar products of some WKB solutions with respect to
the "flux norm" is not invertible. The interest of this procedure lies in its
possible generalization to matrix-valued Hamiltonians, like Bogoliubov-de
Gennes Hamiltonian. It is simplified in the scalar case by using action-angle
variables.
