theorem
  x in Class EqR implies ex y being Element of X st x = Class(EqR,y)
proof
  assume
A1: x in Class EqR;
  then reconsider x as Subset of X;
  consider y such that
A2: y in X and
A3: x = Class(EqR,y) by A1,Def3;
  reconsider y as Element of X by A2;
  take y;
  thus thesis by A3;
end;
