Documenting the ‘how’ part, i.e. the documenting the assumptions that the author had on the context this code runs is important; you guess just enough by greppi

Documenting the ‘how’ part, i.e. the documenting the assumptions that the author had on the context this code runs is important; you guess just enough by grepping calls to the function in the codebase, add add some code that works now… and then the original author or another person adds a call that breaks the assumptions you made, and breaking everything… subtly.