AMG: :: is the prefix or delimiter used to signify namespace qualification in variable and command names. Any sequence of two or more consecutive colons is acceptable.

:: is recognized as the name for the top-level namespace. The true name is empty string, but :: is the alias returned by commands such as [namespace current] to indicate the top-level namespace.

AMG: Preceding a fully qualified variable or command name with any number of ::'s can be used as a hack to obtain a unique string that refers to said variable or command. Just keep adding ::'s until the string is unique, but no matter how many there are, it'll still be interpreted the same.