Star wars recursion

Define a recursive function star$\_$wars$\_$recursive that takes a parameter num$\_$enemy$\_$ships and returns a string command to take down all the enemy ships.

The string command will comprise of alternating beams: $\text{'}*-\text{'}$ and $\text{'}*--\text{'}$

For example:

star$\_$wars$\_$recursive$(3)$ will return $\text{'}*-*--*-\text{'}$

star$\_$wars$\_$recursive$(6)$ will return $\text{'}*-*--*-*--*-*--\text{'}$

star$\_$wars$\_$recursive$(0)$ will return $\text{'}\text{'}$

Explanation of the first and second examples:

star$\_$wars$\_$iteration$(3)$ return value is formed by $\text{'}*-\text{'}+\text{'}*--\text{'}+\text{'}*-\text{'}$

star$\_$wars$\_$iteration$(6)$ return value is formed by $\text{'}*-\text{'}+\text{'}*--\text{'}+\text{'}*-\text{'}+\text{'}*--\text{'}+\text{'}*-\text{'}+\text{'}*--\text{'}$