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: spm_eeg_planarchannelset.m
    path of file: ./repos/spm
    
    the code of the file until where you have to start completion: function planar = spm_eeg_planarchannelset(data)
% Define the planar gradiometer channel combinations

    
function planar = spm_eeg_planarchannelset(data)
% Define the planar gradiometer channel combinations
% FORMAT planar = spm_eeg_planarchannelset(data)
%
% The output cell array contains the horizontal label, vertical label and
% the label after combining the two.
%__________________________________________________________________________

% Vladimir Litvak
% Copyright (C) 2013-2022 Wellcome Centre for Human Neuroimaging


if isa(data, 'cell') && ~isempty(data) && isa(data{1}, 'char')
    input.label = data;
else
    input = data;
end