reserve X,Y,Z for set,
  a,b,c,d,x,y,z,u for object,
  R for Relation,
  A,B,C for Ordinal;
reserve H for Function;
reserve f,g for Function;
reserve M for non empty set;

theorem
  RelIncl X c= [:X,X:]
proof
  set R = RelIncl X;
  let a,b be object;
  assume
A1: [a,b] in R;
  then b in field R by RELAT_1:15;
  then
A2: b in X by Def1;
  a in field R by A1,RELAT_1:15;
  then a in X by Def1;
  hence thesis by A2,ZFMISC_1:87;
end;
