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theorem isfeq (_A1 _A2: set): $ _A1 == _A2 -> (isfun _A1 <-> isfun _A2) $;

Step	Hyp	Ref	Expression
1		id	_A1 == _A2 -> _A1 == _A2
2	1	isfeqd	_A1 == _A2 -> (isfun _A1 <-> isfun _A2)

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp), axs_pred_calc (ax_gen, ax_4, ax_5, ax_6, ax_7, ax_10, ax_12), axs_set (ax_8)