Description: Principle of identity. Theorem *2.08 of WhiteheadRussell p. 101. For
another version of the proof directly from axioms, see idALT . Its
associated inference, idi , requires no axioms for its proof, contrary
to id . Note that the second occurrences of ph in Steps 1 and 2 may
be simultaneously replaced by any wff ps , which may ease the
understanding of the proof. (Contributed by NM, 29-Dec-1992)(Proof
shortened by Stefan Allan, 20-Mar-2006)