
theorem Th4:
  for n be Nat, a be object holds len (n |-> a) = n
  proof
    let n be Nat, a be object;
    set x = n |-> a;
    dom x = Seg len x by FINSEQ_1:def 3;
    hence thesis by FINSEQ_1:6,FUNCOP_1:13;
  end;
