8.3 The . operator
The period operator is not really an operator, as it is always evaluated
during the compilation. It works similarly to the index and arrow operators,
but can only be used on constant values such as modules. In most cases,
modules are simply a clone of a program, in which case the identifiers in
the module will be the same as those in the program. But some modules,
like those created from directories, overload the index operator so that
the identifiers in the module can be something other than those in the program.
For directory modules, the index operator looks in the directory it was
cloned for to find the identifiers.
You can also use the . operator without an identifier preceeding it to
access modules in the same directory as the program itself. For instance,
.my_module.foo would mean 'the identifier foo in the module
my_module in this directory.