let x be object ; :: according to FUNCOP_1:def 6 :: thesis: ( not x in dom <*f*> or <*f*> . x is set )
assume x in dom <*f*> ; :: thesis: <*f*> . x is set
then x in {1} by FINSEQ_1:2, FINSEQ_1:38;
then x = 1 by TARSKI:def 1;
hence <*f*> . x is set ; :: thesis: verum