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: annulus.m
    path of file: ./repos/gptoolbox/mesh
    
    the code of the file until where you have to start completion: function [V,F,bi,bo] = annulus(s,r,varargin)
  % ANNULUS Construct a triangle mesh of a unit annulus.
  % 
  % [V,F] = annulus(s,r)
  % [V,F] = annulus(s,r,'ParameterName',ParameterValue, ...)
  %
  % Inputs:
  %   s  number of samples on the inner ring
  %   r  radius of the inner ring
  %   Optional:
  %     'Flags'  followed by flags to pass to Triangle for meshing. 
  %        {'-q30 -aX'} where X is squared boundary edge length
  %     'R'  followed by outer ring radius {1}
  % Outputs:

    
function [V,F,bi,bo] = annulus(s,r,varargin)
  % ANNULUS Construct a triangle mesh of a unit annulus.
  % 
  % [V,F] = annulus(s,r)
  % [V,F] = annulus(s,r,'ParameterName',ParameterValue, ...)
  %
  % Inputs:
  %   s  number of samples on the inner ring
  %   r  radius of the inner ring
  %   Optional:
  %     'Flags'  followed by flags to pass to Triangle for meshing. 
  %        {'-q30 -aX'} where X is squared boundary edge length
  %     'R'  followed by outer ring radius {1}
  % Outputs:
  %   V  #V by 2 list of mesh vertex positions
  %   F  #F by 3 list of triangle mesh indices
  %   bi  list of vertices on inner boundary
  %   bo  list of vertices on outer boundary
  %   

  flags = nan;
  R = 1;
  params_to_variables = containers.Map( ...
    {'Flags','R'},{'flags','R'});
  v = 1;
  while v <= numel(varargin)
    param_name = varargin{v};
    if isKey(params_to_variables,param_name)
      assert(v+1<=numel(varargin));
      v = v+1;
      % Trick: use feval on anonymous function to use assignin to this workspace
      feval(@()assignin('caller',params_to_variables(param_name),varargin{v}));
    else
      error('Unsupported parameter: %s',varargin{v});
    end