Theorem sbqcom ≪ | index | src | ≫ 

theorem sbqcom {x: nat} (a: nat) (b: wff x): $ a = x -> ([a / x] b <-> b) $;
StepHypRefExpression
1 eqcom 
a = x -> x = a
2 sbq 
x = a -> (b <-> [a / x] b)
3 1, 2 rsyl 
a = x -> (b <-> [a / x] b)
4 3 bicomd 
a = x -> ([a / x] b <-> b)

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp, itru), axs_pred_calc (ax_gen, ax_4, ax_5, ax_6, ax_7, ax_10, ax_12)