https://www.jetbrains.com/help/idea/contract-annotations.html

https://blog.jetbrains.com/idea/2018/05/intellij-idea-2018-2-eap3-advanced-contract-annotations-and-more/
https://blog.jetbrains.com/idea/2014/10/automatic-notnullnullablecontract-inference-in-intellij-idea-14/
https://blog.jetbrains.com/idea/2013/10/better-control-flow-analysis-with-contract-annotations-and-intellij-idea-13/