You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: Callback.m
    path of file: ./repos/BRAPH-2/braph2/src/ds
    
    the code of the file until where you have to start completion: classdef (Sealed=true) Callback < Element
	%Callback is a callback for a property of an element.
	% It is a subclass of <a href="matlab:help Element">Element</a>.
	%
	% A callback refers to a property of another element, which are identified 
	%  by its properties EL and PROP/TAG.
	% For computational efficiency, it is best to use only one instance of Callback
	%  for each prop of an instance of a concrete element EL, using 
	%  EL.getC
    
function prop_number = getPropNumber(varargin)
			%GETPROPNUMBER returns the property number of callback.
			%
			% N = Callback.GETPROPNUMBER() returns the property number of callback.
			%
			% N = Callback.GETPROPNUMBER(CATEGORY) returns the property number of callback
			%  of category CATEGORY
			%
			% Alternative forms to call this method are:
			%  N = CB.GETPROPNUMBER([CATEGORY]) returns the property number of the callback CB.
			%  N = Element.GETPROPNUMBER(CB) returns the property number of 'CB'.
			%  N = Element.GETPROPNUMBER('Callback') returns the property number of 'Callback'.
			%
			% Note that the Element.GETPROPNUMBER(CB) and Element.GETPROPNUMBER('Callback')
			%  are less computationally efficient.
			%
			% See also getProps, Category.
			
			%CET: Computational Efficiency Trick
			
			if nargin == 0
				prop_number = 4;
				return
			end