theorem
  for n being natural Number holds n <= 1 implies n = 0 or n = 1
  proof
    let n be natural Number;
    assume
A1: n <= 1;
    assume that
A2: not n = 0 and
A3: not n = 1;
    n < 0+1 by A1,A3,XXREAL_0:1;
    hence contradiction by A2,Th13;
  end;
