Theorem bitr4 ≪ | index | src | ≫ 

theorem bitr4 (a b c: wff): $ (a <-> b) -> (c <-> b) -> (a <-> c) $;
StepHypRefExpression
1 anl 
(a <-> b) /\ (c <-> b) -> (a <-> b)
2 anr 
(a <-> b) /\ (c <-> b) -> (c <-> b)
3 1, 2 bitr4d 
(a <-> b) /\ (c <-> b) -> (a <-> c)
4 3 exp 
(a <-> b) -> (c <-> b) -> (a <-> c)

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)