reserve E,X,Y,x for set;
reserve A,B,C for Subset of E;
reserve x1,x2,x3,x4,x5,x6,x7,x8,x9,x10 for Element of X;

theorem
  for X being non empty set, A being non empty Subset of X holds A
  is trivial implies ex x being Element of X st A = {x}
proof
  let X be non empty set, A be non empty Subset of X;
  assume A is trivial;
  then ex s being Element of A st A = {s} by Th46;
  hence thesis;
end;
