“…For example, we have seen that the directed percolation universality class quite generically describes the critical properties of phase transitions from active to inactive, absorbing states, which abound in nature. The few exceptions to this rule either require the coupling to another conserved mode [83,84]; the presence, on a mesoscopic level, of additional symmetries that preclude the spontaneous decay A → ∅ as in the so-called parity-conserving (PC) universality class, represented by branching and annihilating random walks A → (n + 1) A with n even, and A + A → ∅ [85] (for recent developments based on nonperturbative RG approaches, see Ref. [86]); or the absence of any first-order reactions, as in the (by now rather notorious) pair contact process with diffusion (PCPD) [87], which has so far eluded a successful field-theoretic treatment [88].…”