pm5.21 Show Code edit
# Two propositions are equivalent if they are both false. Theorem *5.21 of
# [bib/WhiteheadRussell] p. 124.
## Two falsehoods are equivalent
thm (pm5.21 () () (-> (/\ (-. ph) (-. ps)) (<-> ph ps)) (-. ph) (-. ps) pm5.1 con4bid)
(-> (/\ (-. ph) (-. ps)) (<-> ph ps))
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