Octave: create a class variable then access it within class (in constructor)

Question

Maybe it's a trivial question but I would like to know how to access a constant property within the class contructor or a class function in octave. Let's make an example:

classdef Example % < FatherClass
    
    % read-only protected properties
    properties (Constant=true, Access=protected)
        all_levels = {"A","B","C","D"};
    endproperties
    
    % protected properties
    properties(Access=protected)
        level = 'D';
        output = '.';
    endproperties

    methods(Access=public)
        function obj = Example (level,outputfilepath)

            if any(strcmp(all_levels,level))
                obj.level = level;
            else
                error ("possible levels are: A B C D");
            endif

            obj.output = outputfilepath

        endfunction

    endmethods
end

running this class example I receive the error:

error: 'all_levels' undefined near line 12, column 12
error: called from
    Example at line 12 column 13

So, I've tried something like

if any(strcmp(obj.all_levels,level))
     obj.level = level;

With the same result, also defining a getter:

methods (Static = true)
  function lvs = gel_levels()
     lvs = all_levels
  endfunction
endmethods

...

methods(Access=public)
  function obj = Example (obj,level,outputfilepath)
    all_levels = get_levels()
    % disp(all_levels)
    if any(strcmp(all_levels,level))
       obj.level = level;
    else
       error ("possible levels are: A B C D");
    endif
       obj.output = outputfilepath
  endfunction
endmethods

Sorry but I'm quite new to octave and I haven't found any example about this. What I'm trying to accomplish is a simple class variable