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: ekman.m
    path of file: ./repos/CDT/cdt
    
    the code of the file until where you have to start completion: function varargout = ekman(lat,lon,u10,v10,varargin) 
% ekman estimates the classical Ekman transport and upwel
    
function below which calculates Cd by Lüpkes and Birnbaum, 2005. 
   end
   
   % Check for drag coefficient: 
   tmp = strcmpi(varargin,'Cd'); 
   if any(tmp) 
      Cd = varargin{find(tmp)+1}; 
      assert(ice==false,'Input error: If you enter sea ice concentration you cannot also specify a drag coefficient. Pick one and try again.')
   end