  While revisiting Klein-Gordon relativistic quantum equation for spin-0
particles, we predicted that $\hbar$ reverses its sign for negative energies,
and formulated a universal symmetry rule, whereby all the parameters that
couple particles to external fields reverse their sign along with $\hbar$ at a
particle$\leftrightarrow$antiparticle transformation; this in particular
implies anti-gravitation between matter and antimatter. Our results suggest
that the $\hbar$-conjugation principle and related invariance may replace
$CPT$-invariance in general relativistic quantum mechanics.
