Definition
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[227]For convenience, the \(aβ b\) connective is used to indicate that \(a\) is a subset of \(b\); formally this is defined by
\[ β x, xβ a β xβ b~ ~ . \]
\(bβ a\) is equivalent to \(aβ b\).
Obviously \(a=b \iff ((aβ b) β§ (bβ a) )\). Note that \(aβ a\).