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: histvol.m
    path of file: ./repos/advanced_fMRI_course/Practical#1
    
    the code of the file until where you have to start completion: function [n, x] = histvol(V, nbins)
    % Create Histogram of an image volume
    % FORMAT [n, x]=histvol(V, nbins)
    % V     - mapped image volume (see spm_vol)
    % nbins - number of bins to use.
    % n     - number of counts in each bin
    % x     - position of bin centres
    % _______________________________________________________________________

    
function [n, x] = histvol(V, nbins)
    % Create Histogram of an image volume
    % FORMAT [n, x]=histvol(V, nbins)
    % V     - mapped image volume (see spm_vol)
    % nbins - number of bins to use.
    % n     - number of counts in each bin
    % x     - position of bin centres
    % _______________________________________________________________________
    % @(#)JohnsGems.html    1.42 05/02/02

    if nargin == 1
        nbins = 256;
    end