reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for XFinSequence;
reserve D for set;
reserve i for Nat;
reserve m for Nat,
        D for non empty set;
reserve l for Nat;
reserve M for Nat;
reserve m,n for Nat;
reserve x1,x2,x3,x4 for object;

theorem
 len<%x1,x2,x3,x4%> = 4
  proof
   thus len<%x1,x2,x3,x4%>
       = len<%x1,x2,x3%> + 1 by Th72
      .= 3 + 1 by Th36
      .= 4;
  end;
