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: aas_checkreg.m
    path of file: ./repos/automaticanalysis/aa_engine
    
    the code of the file until where you have to start completion: % FSL-like overlay + SPM orthview video (wiht structural)
    
function aas_checkreg(aap,varargin)

%% Parse arguments

index{1} = varargin{1};
if isnumeric(varargin{2})
    index{2} = varargin{2};
    iarg0 = 4;
else
    iarg0 = 3;
end

if isnumeric(index{1})
    switch numel(index)
        case 1 % subject level
            index{2} = index{1};
            index{1} = 'subject';
        case 2 % session level
            index{2} = cell2mat(index);
            if ~isempty(strfind(aap.tasklist.currenttask.domain,'session'))
                index{1} = aap.tasklist.currenttask.domain;
            else
                index{1} = 'session';
            end