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: spm_Xpdf.m
    path of file: ./repos/spm
    
    the code of the file until where you have to start completion: function f = spm_Xpdf(x,v)
% Probability Density Function (PDF) of Chi-Squared distribution
% FORMAT f = spm_Xpdf(x,v)
%
% x - Chi-squared variate
% v - degrees of freedom (v>0, non-integer d.f. accepted)
% f - PDF at x of Chi-squared distribution with v degrees of freedom
%__________________________________________________________________________
%
% spm_Xpdf implements the Probability Density Function of the 
% Chi-squared distributions.
%
% Definition:
%--------------------------------------------------------------------------
% The Chi-squared distribution with v degrees of freedom is defined for
% positive integer v and x in [0,Inf), and has Probability Distribution
% Function (PDF) f(x) given by: (See Evans et al., Ch8)
%
%           x^((v-2)/2) exp(-x/2)
%    f(x) = ---------------------
%           2^(v/2) * gamma(v/2)
%
% Variate relationships: (Evans et al., Ch8 & Ch18)
%--------------------------------------------------------------------------
% The Chi-squared distribution with v degrees of freedom is equivalent
% to the Gamma distribution with scale parameter 1/2 and shape parameter v/2.
%
% Algorithm:
%--------------------------------------------------------------------------
% Using routine spm_Gpdf for Gamma distribution, with appropriate parameters.
%
% References:
%--------------------------------------------------------------------------
% Evans M, Hastings N, Peacock B (1993)
%       "Statistical Distributions"
%        2nd Ed. Wiley, New York
%
% Abramowitz M, Stegun IA, (1964)
%       "Handbook of Mathematical Functions"
%        US Government Printing Office
%
% Press WH, Teukolsky SA, Vetterling AT, Flannery BP (1992)

    
function f = spm_Xpdf(x,v)
% Probability Density Function (PDF) of Chi-Squared distribution
% FORMAT f = spm_Xpdf(x,v)
%
% x - Chi-squared variate
% v - degrees of freedom (v>0, non-integer d.f. accepted)
% f - PDF at x of Chi-squared distribution with v degrees of freedom
%__________________________________________________________________________
%
% spm_Xpdf implements the Probability Density Function of the 
% Chi-squared distributions.
%
% Definition:
%--------------------------------------------------------------------------
% The Chi-squared distribution with v degrees of freedom is defined for
% positive integer v and x in [0,Inf), and has Probability Distribution
% Function (PDF) f(x) given by: (See Evans et al., Ch8)
%
%           x^((v-2)/2) exp(-x/2)
%    f(x) = ---------------------
%           2^(v/2) * gamma(v/2)
%
% Variate relationships: (Evans et al., Ch8 & Ch18)
%--------------------------------------------------------------------------
% The Chi-squared distribution with v degrees of freedom is equivalent
% to the Gamma distribution with scale parameter 1/2 and shape parameter v/2.
%
% Algorithm:
%--------------------------------------------------------------------------
% Using routine spm_Gpdf for Gamma distribution, with appropriate parameters.
%
% References:
%--------------------------------------------------------------------------
% Evans M, Hastings N, Peacock B (1993)
%       "Statistical Distributions"
%        2nd Ed. Wiley, New York
%
% Abramowitz M, Stegun IA, (1964)
%       "Handbook of Mathematical Functions"
%        US Government Printing Office
%
% Press WH, Teukolsky SA, Vetterling AT, Flannery BP (1992)
%       "Numerical Recipes in C"
%        Cambridge
%
%__________________________________________________________________________

% Andrew Holmes
% Copyright (C) 1993-2022 Wellcome Centre for Human Neuroimaging


%-Check enough arguments
%--------------------------------------------------------------------------
if nargin<2, error('Insufficient arguments'), end