A5: less_eq_dom f,a c= X

proof

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in less_eq_dom f,a or x in X )
assume A6: x in less_eq_dom f,a ; :: thesis: x in X
A7: x in dom f by A6, Def13;
A8: dom f c= X by RELAT_1:def 18;
thus x in X by A7, A8; :: thesis: verum

end;

thus less_eq_dom f,a is Subset of X by A5; :: thesis: verum