reserve Omega, I for non empty set;
reserve Sigma for SigmaField of Omega;
reserve P for Probability of Sigma;
reserve D, E, F for Subset-Family of Omega;
reserve  B, sB for non empty Subset of Sigma;
reserve b for Element of B;
reserve a for Element of Sigma;
reserve p, q, u, v for Event of Sigma;
reserve n, m for Element of NAT;
reserve S, S9, X, x, y, z, i, j for set;

theorem Th2:
  for r being Real st r*r=r holds r=0 or r=1
proof
  let r be Real;
  assume r*r=r;
  then r*(r-1)=0;
  then r=0 or r-1=0 by XCMPLX_1:6;
  hence thesis;
end;
