How is arc defined in TikZ?
I couldn't a decent definition a TikZ book or PGF manual. Can someone please explain?

In TikZ, the `arc` command doesn't its own definition -- it instead makes use of the `pic` mechanism to draw arcs in a parametric fashion. Here's a brief explanation:

1. First, it's important to understand that TikZ and PGF are not just standalone packages, but rather libraries that can be used with LaTeX or other TeX engines. They provide a large set of features for creating graphics, including commands
How do I add a validation to make sure the date string being passed to the method is in the ffg. format:
```'YYYY-MM-DD'
```
if it's not, method should raise some sort of error

To add validation to ensure the date string is in the format 'YYYY-MM-DD', you can use a library like `java.time` and check if the string can be parsed to a `LocalDate` object. If it fails, throw an exception. Here's an example of how to do it in a method:

```java
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public void methodName(String dateString) {
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");