return this environment, if it matches before
any other criterion is satisfied. The default, the option
topLevelEnvironment, is set by sys.source,
which treats a specific environment as the top level environment.
Supplying the argument as NULL means it will never match.
Details
topenv returns the first top level environment found when
searching envir and its enclosing environments. An environment is
considered top level if it is the internal environment of a namespace,
a package environment in the search path, or .GlobalEnv.