Theorem elArraylen ≪ | index | src | ≫ 

theorem elArraylen (A: set) (l n: nat): $ l e. Array A n -> len l = n $;
StepHypRefExpression
1 elArray 
l e. Array A n <-> l e. List A /\ len l = n
2 anr 
l e. List A /\ len l = n -> len l = n
3 1, 2 sylbi 
l e. Array A n -> len l = n

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_11, ax_12), axs_set (elab, ax_8), axs_the (theid, the0), axs_peano (peano2, addeq, muleq)