reserve x for set,
  D for non empty set,
  k,n,m,i,j,l for Nat,
  K for Field;
reserve n,i,j for Nat;

theorem Th74:
  len Base_FinSeq(n,i) = n
proof
  len((n |-> 0)+*(i,1)) = len (n |-> 0) by FUNCT_7:97
    .= n by CARD_1:def 7;
  hence thesis;
end;
