reserve a,b,c,k,m,n for Nat;
reserve i,j,x,y for Integer;
reserve p,q for Prime;
reserve r,s for Real;

theorem Th6:
  r <= s implies (r,s are_twin iff s-r = 2)
  proof
    assume r <= s;
    then s-r >= r-r by XREAL_1:7;
    hence thesis by ABSVALUE:def 1;
  end;
