theorem Th60:
  ex y st x = y+y
proof
  set O2 = one+one;
  O2 = 1+^1 by Th58;
  then O2 <> {} by ORDINAL3:26;
  then consider z such that
A1: O2*'z = 1 by Th54;
  take y = z*'x;
  thus x = one*'x by Th53
    .= O2*'y by A1,Th52
    .= one*'y+one*'y by Th57
    .= y+one*'y by Th53
    .= y+y by Th53;
end;
