let S, X be non empty set ; :: thesis: for R being Relation of X st R is symmetric holds
R is_symmetric_in S
let R be Relation of X; :: thesis: ( R is symmetric implies R is_symmetric_in S )
assume R is symmetric ; :: thesis: R is_symmetric_in S
then A1: R is_symmetric_in field R ;
let x, y be object ; :: according to RELAT_2:def 3 :: thesis: ( not x in S or not y in S or not [x,y] in R or [y,x] in R )
assume ( x in S & y in S ) ; :: thesis: ( not [x,y] in R or [y,x] in R )
assume A2: [x,y] in R ; :: thesis: [y,x] in R
then ( x in field R & y in field R ) by RELAT_1:15;
hence [y,x] in R by A1, A2; :: thesis: verum