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: ft_headmodel_singlesphere.m
    path of file: ./repos/spm12/external/fieldtrip/forward
    
    the code of the file until where you have to start completion: function headmodel = ft_headmodel_singlesphere(mesh, varargin)

% FT_HEADMODEL_SINGLESPHERE creates a volume conduction model of the
% head by fitting a spherical model to a set of points that describe
% the head surface.
%
% For MEG this implements Cuffin BN, Cohen D.  "Magnetic fields of
% a dipole in special volume conductor shapes" IEEE Trans Biomed Eng.
% 1977 Jul;24(4):372-81.
%
% Use as
%   headmodel = ft_headmodel_singlesphere(mesh, ...)

    
function headmodel = ft_headmodel_singlesphere(mesh, varargin)

% FT_HEADMODEL_SINGLESPHERE creates a volume conduction model of the
% head by fitting a spherical model to a set of points that describe
% the head surface.
%
% For MEG this implements Cuffin BN, Cohen D.  "Magnetic fields of
% a dipole in special volume conductor shapes" IEEE Trans Biomed Eng.
% 1977 Jul;24(4):372-81.
%
% Use as
%   headmodel = ft_headmodel_singlesphere(mesh, ...)
%
% Optional arguments should be specified in key-value pairs and can include
%   conductivity     = number, conductivity of the sphere
%
% See also FT_PREPARE_VOL_SENS, FT_COMPUTE_LEADFIELD

% FIXME document both EEG and MEG case

% Copyright (C) 2012-2013, Donders Centre for Cognitive Neuroimaging, Nijmegen, NL
%
% This file is part of FieldTrip, see http://www.fieldtriptoolbox.org
% for the documentation and details.
%
%    FieldTrip is free software: you can redistribute it and/or modify
%    it under the terms of the GNU General Public License as published by
%    the Free Software Foundation, either version 3 of the License, or
%    (at your option) any later version.
%
%    FieldTrip is distributed in the hope that it will be useful,
%    but WITHOUT ANY WARRANTY; without even the implied warranty of
%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%    GNU General Public License for more details.
%
%    You should have received a copy of the GNU General Public License
%    along with FieldTrip. If not, see <http://www.gnu.org/licenses/>.
%
% $Id$

% get the optional arguments
conductivity = ft_getopt(varargin, 'conductivity', 1);

if any(strcmp(varargin(1:2:end