Fermion annihilation operator in matrix representation for a indexed
       particle in a bounded N-particles fermion fock space

def destruct(particles, index):
    """Fermion annihilation operator in matrix representation for a indexed
       particle in a bounded N-particles fermion fock space"""

    mat = np.zeros((2**particles, 2**particles))

    flipper = 2**index
    for i in range(2**particles):
        ispin = btest(i, index)
        if ispin == 1:
            mat[i ^ flipper, i] = phase(i, index)
    return csr_matrix(mat)