theorem Th63:
 for X being non empty set, A being Singleton of X
  ex x being Element of X st A = {x}
  proof let X be non empty set, A be Singleton of X;
    consider x being object such that
A1:   A = {x} by ZFMISC_1:131;
    x in A by A1,TARSKI:def 1;
    then reconsider x as Element of X;
   take x;
   thus thesis by A1;
  end;
