let y, x be set ; :: thesis: for R being Relation holds
( y in Class R,x iff [x,y] in R )
let R be Relation; :: thesis: ( y in Class R,x iff [x,y] in R )
thus ( y in Class R,x implies [x,y] in R ) :: thesis: ( [x,y] in R implies y in Class R,x )

proof

assume y in Class R,x ; :: thesis: [x,y] in R
then ex z being set st
( [z,y] in R & z in {x} ) by RELAT_1:def 13;
hence [x,y] in R by TARSKI:def 1; :: thesis: verum

end;

assume A1: [x,y] in R ; :: thesis: y in Class R,x
x in {x} by TARSKI:def 1;
hence y in Class R,x by A1, RELAT_1:def 13; :: thesis: verum