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: OPdiffusion.m
    path of file: ./repos/IRtools/OPcodes
    
    the code of the file until where you have to start completion: function w = OPdiffusion(u,R,S,Tsteps,tflag)
% OPdiffusion The forward computation and its adjoint for PRdiffusion
%
% w = OPdiffusion(u,R,S,Tfinal,Tsteps,tflag)
%
% Performs the forward computation or returns the problem dimensions.
%
% Input: u      - the vector to be operated upon
%        R,S    - two matrices needed for the iterations
%        Tfinal - diffusion time
%        Tsteps - number of time steps
%        tflag  - string that determines the computation:
%                 'notransp', 'transp' or 'size'

% Based on original code by Allan P. Engsig-Karup, DTU Compute.

% Silvia Gazzola, University of Bath
% Per Christian Hansen, Technical University of Denmark
% James G. Nagy, Emory University
% April, 2018.

% This file is part of the IR Tools package and is distributed under the 
% 3-Clause BSD Licence. A separate license file should be provided as part 
% of the package.

n = size(R,1);

if strcmpi(tflag,'size')
    w(1) = n;
    w(2) = n;

    
function w = OPdiffusion(u,R,S,Tsteps,tflag)
% OPdiffusion The forward computation and its adjoint for PRdiffusion
%
% w = OPdiffusion(u,R,S,Tfinal,Tsteps,tflag)
%
% Performs the forward computation or returns the problem dimensions.
%
% Input: u      - the vector to be operated upon
%        R,S    - two matrices needed for the iterations
%        Tfinal - diffusion time
%        Tsteps - number of time steps
%        tflag  - string that determines the computation:
%                 'notransp', 'transp' or 'size'

% Based on original code by Allan P. Engsig-Karup, DTU Compute.

% Silvia Gazzola, University of Bath
% Per Christian Hansen, Technical University of Denmark
% James G. Nagy, Emory University
% April, 2018.

% This file is part of the IR Tools package and is distributed under the 
% 3-Clause BSD Licence. A separate license file should be provided as part 
% of the package.

n = size(R,1);

if strcmpi(tflag,'size')
    w(1) = n;
    w(2) = n;
elseif strcmpi(tflag,'notransp')
    w = u;
    for i = 1:Tsteps
        w = R\(S*w);
    end