reserve x,y,z for object,
  i,j,n,m for Nat,
  D for non empty set,
  s,t for FinSequence,
  a,a1,a2,b1,b2,d for Element of D,
  p, p1,p2,q,r for FinSequence of D;

theorem Th5:
  {} is tabular
proof
    take n=0;
    let x;
    assume x in rng ({});
    hence ex t st t=x & len t=n;
end;
