What does $\alpha_r\supset\beta_r$ mean in the tolerance definition of System Z?

Question

I'm reading up on System Z introduced by Judea Pearl (in System Z: a natural ordering of defaults with tractable applications to nonmonotonic reasoning). A central definition is that of tolerance of a subset $R'$ of a rule set $R$ for a rule $r = \alpha_r\to\beta_r$ (denoted by $T(r\ \vert\ R')$). Tolerance of $R'$ for $r$ is defined to be the set of satisfiable formulas $$(\alpha_r\land\beta_r)\bigcup_{r'\in R'}(\alpha_{r'}\supset\beta_{r'})$$ (see page 2 of the article by Pearl).

I don't understand what $\alpha_{r'}\supset\beta_{r'}$ is supposed to mean. Can anyone explain?