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: diff.m
    path of file: ./repos/chebfun/@diskfun
    
    the code of the file until where you have to start completion: function f = diff( f, varargin )
%DIFF   Derivative of a diskfun in Cartesian coordinates.
%
%   F = DIFF( F ) computes the first derivative of F with respect to x.
%
%   F = DIFF( F, DIM )  computes the first derivative of F. If DIM = 1, the
%  
    
function f = onediff(f, dim)

% Make sure f has as short a length as possible.
f=simplify(f); 

% We are going to work at the tech level to make things faster.
[C, ~, R] = cdr( f );

% Do everything with even length columns since then no special
% modifications are required for dividing by rho series expansion.
parity = mod(length(C),2);
n = length(C)+parity;
m = length(R);

% The variable coefficients in the definitions of the derivatives means
% that the length of the columns and rows will increase by one wave number
% after taking the derivatives with respect to x and y. 
m = m+2;
n = n+2;

% Matrices for multiplying by sin(theta), cos(theta) and 1/r in coefficient space.
Msinm = .5i*spdiags(ones(m,1)*[-1,1],[-1 1],m,m);
Mcosm = .5*spdiags(ones(m,1)*[1,1],[-1 1],m,m);
Mn = ultraS.multmat(n, [0;1], 0);

% Work at the tech level to make things faster.
ctechs = C.funs{1}.onefun;
rtechs = R.funs{1}.onefun;

% Alias to get the extra zeros in place.
ctechs.coeffs = ctechs.alias(ctechs.coeffs, n); 
rtechs.coeffs = rtechs.alias(rtechs.coeffs, m); 

% Compute the derivatives.
dCdr = diff(ctechs);
dRdth = diff(rtechs)/pi;

% 1/r*col
rinv = Mn \ ctechs.coeffs; 

if (dim==1) %d/dx 

    %CDR for -1/r.*sin(th).*d/dth
    C1 = rinv; 
    R1 = -Msinm*dRdth.coeffs;
    
    %CDR for cos(th).*d/dr
    C2 = dCdr.coeffs;
    R2 = Mcosm*rtechs.coeffs;
    
else  %d/dy
    
    %CDR for 1/r.*cos(th).*d/dth
    C1 = rinv; 
    R1 = Mcosm*dRdth.coeffs;
    
    %CDR for sin(th).*d/dr
    C2 = dCdr.coeffs;
    R2 = Msinm*rtechs.coeffs;
end