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: sqi_power_level_crossing.m
    path of file: ./repos/OSET/matlab/tools/sqi
    
    the code of the file until where you have to start completion: function [index, rank, yy] = sqi_power_level_crossing(x, w, th, varargin)

    
function [index, rank, yy] = sqi_power_level_crossing(x, w, th, varargin)
% Channel selection based on the number of level crossings of the signal power envelope
%
% Usage:
%   [index, rank, yy] = sqi_power_level_crossing(x, w, th, ranking_mode, normalize)
%
% Inputs:
%   x: ECG signal (rows represent channels, and columns represent samples)
%   w: window size (number of samples) for the moving average filter to compute the signal power envelope
%   th: threshold value for identifying the power envelope level crossings (% of power envelope maximum)
%   ranking_mode (optional): 'ascend