reserve x,y,t for Real;

theorem Th28:
  x^2<1 & y^2<1 implies x*y<>1
proof
  assume
A1: x^2<1 & y^2<1;
  assume x*y=1;
  then x*y*(x*y)=1;
  then (x*x)*(y*y)=1;
  hence contradiction by A1,Lm2;
end;
