1. Test the empty string
1. Allow argument with one number
1. Allow two comma delimited numbers
2. Our add already allows more than two numbers
3. Allow new lines as delimiters
4. Support different delimiters
4. Refactor
5. Throw exception if numbers are negative
5. Refactor
6. Only sum numbers less than 1001
7. Allow delimiters of any length
8. Allow multiple delimiters