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;

theorem Th72:
  len(p^<%x%>) = len p + 1
 proof
  thus len(p^<%x%>) = len p + len<%x%> by Def3
      .= len p + 1 by Th30;
 end;
