Given that the following code has executed, what is the value of mystery?

pets $=\{\text{'}$dogs$\text{'}$: $[\text{'}$Rocket$\text{'},$ 'Molly', 'Trixie'$],$

'cats': $[\text{'}$Hobbes$\text{'},$ 'Motley'$]\}$

pets$[\text{'}$cats$\text{'}].$append$(\text{'}$Jet$\text{'})$

pets$[\text{'}$snakes$\text{'}]=[\text{'}$Squiggy$\text{'},$ 'Slinky', 'Striker'$]$

pets$[\text{'}$cats$\text{'}].$append$(\text{'}$Jenny$\text{'})$

mystery $=$ pets$[\text{'}$cats$\text{'}][-1][-1]$