I'm currently learning Objective-C and working my way through a book which contains lots of exercises. At the beginning of the book I set up an OS X project command line project and have been working in the `main.m` file, block commenting prior exercises, but it's becoming tiresome working in such a long file, constantly commenting and un-commenting the exercise code I want to build. I'd like to create separate files in the Navigator for each exercise so they are easy to find. How would I go about doing this without causing issues with the file in the Products folder?

I'm currently getting this red alert when I try to build after adding a new `.m` file:

```
Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

```