- almost exclusively used for variables or types
- indicates the current context (usually the current type when used as the type of a newly created value initialized)
- this name is unimportant and can be safely left 'anonymous'. '_' is never a true variable.
- most often used for implicit arguments of closures, which start with @1, @0 would be the closure itself.
- used as a placeholder where the name (often a type) will not be specified.
Further use of this name indicates an identical type.