reserve a,b,c,k,k9,m,n,n9,p,p9 for Nat;
reserve i,i9 for Integer;

theorem Th5:
  m^2 = n^2 implies m = n
proof
  assume
A1: m^2 = n^2;
  per cases by A1,SQUARE_1:40;
  suppose
    m = n;
    hence thesis;
  end;
  suppose
A2: m = -n;
    then m = -0;
    hence thesis by A2;
  end;
end;
