let x, y, X be set ; :: thesis: for R being Relation of X st [x,y] in R holds
( x in X & y in X )
let R be Relation of X; :: thesis: ( [x,y] in R implies ( x in X & y in X ) )
assume [x,y] in R ; :: thesis: ( x in X & y in X )
then ( x in dom R & y in rng R ) by RELAT_1:20;
hence ( x in X & y in X ) ; :: thesis: verum