I'm a new developer considering using Hack. Can you tell me what Hack is, what its main advantages are, and provide the exact command-line steps to install HHVM and Hack on a Debian 9 Stretch system?
Navigate to 'hacklang.org'. Identify and extract the core description of Hack and its main advantages from the homepage. Click on the 'Install' link (ID 10) on the homepage. On the 'Getting Started' page, click on the 'HHVM installation page' link (ID 719). On the 'Installation: Introduction' page, click on the 'Several versions of Debian and Ubuntu' link (ID 156). Locate the installation instructions for 'Debian 9 Stretch' on the 'Installation: Linux' page. Extract all command-line steps required for installing HHVM and Hack on Debian 9 Stretch. Synthesize the description, advantages, and installation steps into a concise answer.
The agent successfully navigates to 'hacklang.org'. The agent states that Hack is 'an object-oriented programming language for building reliable websites at epic scale' and identifies 'Fast Development', 'Type Checking', and 'Built for HHVM' as its main advantages. The agent successfully navigates through the installation links to reach the 'Installation: Linux' page. The agent provides the following exact command-line steps for Debian 9 Stretch installation:
1. `wget -O - https://dl.hhvm.com/conf/hhvm.gpg.key | sudo apt-key add -`
2. `echo deb https://dl.hhvm.com/debian stretch main | sudo tee /etc/apt/sources.list.d/hhvm.list`
3. `sudo apt-get update`
4. `sudo apt-get install hhvm`