let X be set ; :: thesis: for P, R being Relation st P c= R holds
P | X c= R | X
let P, R be Relation; :: thesis: ( P c= R implies P | X c= R | X )
assume A1: P c= R ; :: thesis: P | X c= R | X
let x be object ; :: according to RELAT_1:def 3 :: thesis: for b being object st [x,b] in P | X holds
[x,b] in R | X
let y be object ; :: thesis: ( [x,y] in P | X implies [x,y] in R | X )
assume [x,y] in P | X ; :: thesis: [x,y] in R | X
then ( [x,y] in P & x in X ) by Def9;
hence [x,y] in R | X by A1, Def9; :: thesis: verum