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: doppler_shift.m
    path of file: ./repos/gps-stack-sim/lib
    
    the code of the file until where you have to start completion: function [ dop_f ] = doppler_shift( vs, vr, c, fo )
%DOPPLER_SHIFT calculates output frecuency for doppler effect system
%c: wave speed
%vr: receiver speed
%vs: source speed
%fo: original wave 

    
function [ dop_f ] = doppler_shift( vs, vr, c, fo )
%DOPPLER_SHIFT calculates output frecuency for doppler effect system
%c: wave speed
%vr: receiver speed
%vs: source speed
%fo: original wave 
%
%dop_f: new frequency

dop_f = (c + vr) .* fo ./ (c + vs);

end