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: mdm_pa_ind_from_xps.m
    path of file: ./repos/md-dmri/mdm
    
    the code of the file until where you have to start completion: function [a_ind, c_list, id_ind] = mdm_pa_ind_from_xps(xps, opt)
% function [a_ind, c_list, id_ind] = mdm_pa_ind_from_xps(xps, opt)
%
% Volumes with identical rotations is defined from xps.a_ind
%
% If this does not exist, we try to compute it, but beware of errors in
% this step

if (nargin < 2), opt = []; end 
opt = mdm_opt
    
function is a can of worms...
    if (isfield(xps, 'b') && (isfield(xps, 'b_delta')))
        
        % define step in b and b_delta
        db        = opt.mdm.pa.db * 1e-9; % units of um2/ms
        db_delta2 = opt.mdm.pa.db_delta2;
        
        b = xps.b * 1e-9;
        b_delta2 = xps.b_delta.^2 .* sign(xps.b_delta);
        
        x = [b / db b_delta2 / db_delta2];
        [~,~,a_ind] = uniquetol(x, 1/max(abs(x(:))), 'byrows', 1);

    elseif (isfield(xps, 'b'))
        
        % just do pa of b
        x = xps.b / opt.mdm.pa.db;
        [~,~,a_ind] = uniquetol(x, 1/max(abs(x)));
        
    else
        error('insufficient information to compute power average indices');
    end