reserve i,j,x,y for object,
  f,g for Function;
reserve T,T1 for finite Tree,
  t,p for Element of T,
  t1 for Element of T1;

theorem Th24:
  for D being finite set st card D = 1 holds ex x being Element of D st D={x}
proof
  let D be finite set;
  assume card D = 1;
  then card D = 0+1;
  then consider x being Element of D,C being Subset of D such that
A1: D = C \/ { x } and
A2: card C = 0 by Th23;
  take x;
  C = {} by A2;
  hence thesis by A1;
end;
