Yes, they're very common in build.sbt files. How else would you add library dependencies?

I know exactly what they do; the underlying reasoning is very consistent, maybe even elegant. But if you don't know and need to understand/modify a build.sbt file, it's very tricky to find out.

> How else would you add library dependencies?

Like this?

  libraryDependencies ++= Seq(...)

Ok, so what's the difference between that and

    libraryDependencies :++= Seq(...)

? Because I've definitely seen that in some build.sbt files.

This doesn't exist:

    error: value :++= is not a member of sbt.TaskKey[Seq[sbt.ModuleID]]