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: gaussf.m
    path of file: ./repos/diplib/dipimage
    
    the code of the file until where you have to start completion: %GAUSSF   Gaussian filter
%
% SYNOPSIS:
%  image_out = gaussf(image_in,sigma,method,boundary_condition,truncation)
%
% PARAMETERS:
%  sig
    
function calls the DIPlib function dip::Derivative (which calls dip::GaussFIR,
%  dip::GaussIIR and dip::GaussFT) and dip::CreateGauss.

% (c)2017-2018, Cris Luengo.
% Based on original DIPlib code: (c)1995-2014, Delft University of Technology.
% Based on original DIPimage code: (c)1999-2014, Delft University of Technology.
%
% Licensed under the Apache License, Version 2.0 (the "License");
% you may not use this file except in compliance with the License.
% You may obtain a copy of the License at
%
%    http://www.apache.org/licenses/LICENSE-2.0
%
% Unless required by applicable law or agreed to in writing, software
% distributed under the License is distributed on an "AS IS" BASIS,
% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
% See the License for the specific language governing permissions and
% limitations under the License.

function image_out = gaussf(image_in,sigma,method,varargin)
% The code below looks a little silly, but it's an easy way to not parse input arguments twice.
if nargin < 2
   image_out = dip_filtering('derivative',image_in);
elseif nargin < 3
   image_out = dip_filtering('derivative',image_in,0,sigma);
else
   if ~ischar(method) && ~isvector(method)
      error('METHOD argument must be a string');
   end