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: FBP.m
    path of file: ./repos/TIGRE/MATLAB/Algorithms
    
    the code of the file until where you have to start completion: function [res,errorL2]=FBP(proj,geo,angles,varargin)
%TODO docs FBP
% 
%
%--------------------------------------------------------------------------
%--------------------------------------------------------------------------
% This file is part of the TIGRE Toolbox
% 
% Copyright (c) 2015, University of Bath and 
%                     CERN-European Organization for Nuclear Research
%                     All rights reserved.
%
% License:            Open Source under BSD. 
%                     See the full license at
%                     https://github.com/CERN/TIGRE/blob/master/LICENSE
%                     and
%                     https://www.mathworks.com/matlabcentral/fileexchange/view_license?file_info_id=35548
%
% Contact:            tigre.toolbox@gmail.com
% Codes:              https://github.com/CERN/TIGRE/
% Coded by:           Kyungsang Kim, modified by Ander Biguri 
%--------------------------------------------------------------------------


    
function [res,errorL2]=FBP(proj,geo,angles,varargin)
%TODO docs FBP
% 
%
%--------------------------------------------------------------------------
%--------------------------------------------------------------------------
% This file is part of the TIGRE Toolbox
% 
% Copyright (c) 2015, University of Bath and 
%                     CERN-European Organization for Nuclear Research
%                     All rights reserved.
%
% License:            Open Source under BSD. 
%                     See the full license at
%                     https://github.com/CERN/TIGRE/blob/master/LICENSE
%                     and
%                     https://www.mathworks.com/matlabcentral/fileexchange/view_license?file_info_id=35548
%
% Contact:            tigre.toolbox@gmail.com
% Codes:              https://github.com/CERN/TIGRE/
% Coded by:           Kyungsang Kim, modified by Ander Biguri 
%--------------------------------------------------------------------------

% Assertion exploiting lazy evaluation
if isfield(geo,'mode') && ~strcmpi(geo.mode,'parallel')
     assert(false,'Only use FBP for parallel beam CT')
end