reserve a,b,c,x,y,z for Real;

theorem
  a^2 <= 1 implies -1 <= a & a <= 1
proof
  assume a^2<=1;
  then a^2-1^2<=1^2-1^2 by XREAL_1:9;
  then (a-1)*(a+1)<=0;
  hence thesis by XREAL_1:93;
end;
