reserve a,b,c for Integer;
reserve i,j,k,l for Nat;
reserve n for Nat;
reserve a,b,c,d,a1,b1,a2,b2,k,l for Integer;
reserve p,p1,q,l for Nat;

theorem Th28:
  2 is prime
proof
  thus 2>1;
  let n be Nat;
  assume
A1: n divides 2;
  then n <= 2 by Th27;
  then n = 0 or ... or n = 2;
  hence thesis by A1;
end;
