Write a function named retain$\_$every$\_$other$\_$line that has two

parameters: the name of an input file and the name of an output file. The function should

write every other line of the input file to the output file. For example, if the input file contains

five lines, the output file should end up with three separate lines: the first line of the input file,

the third line of the input file and the fifth line of the input file.

You may assume that the input file exists. Here is a sample call to the function:

retain$\_$every$\_$other$\_$line$("$input$.$txt$",$ "output.txt$")$