reconsider mk = mk as Element of NAT by ORDINAL1:def 12;
take mk ; :: thesis: ex f being FinSequence of NAT st
( f = x `2_3 & mk = f /. 1 )
take <*mk*> ; :: thesis: ( <*mk*> = x `2_3 & mk = <*mk*> /. 1 )
thus ( <*mk*> = x `2_3 & mk = <*mk*> /. 1 ) by A1, FINSEQ_4:16; :: thesis: verum