To start with, all the symbols that can be found in a given package using are said to be accessible in that package. In other words, the accessible symbols in a package are those that can be referred to with unqualified names when the package is current.

The other way a symbol can be accessible in a package is if the package inherits it. A package inherits symbols from other packages by using the other packages. Only external symbols in the used packages are inherited. A symbol is made external in a package by exporting it. In addition to causing it to be inherited by using packages, exporting a symbol also—as you saw in the previous section—makes it possible to refer to the symbol using a single-colon qualified name.

An existing symbol can be imported into another package by adding it to the package’s name-to-symbol table. Thus, the same symbol can be present in multiple packages. Sometimes you’ll import symbols simply because you want them to be accessible in the importing package without using their home package. Other times you’ll import a symbol because only present symbols can be exported or be shadowing symbols. For instance, if a package needs to use two packages that have external symbols of the same name, one of the symbols must be imported into the using package in order to be added to its shadowing list and make the other symbol inaccessible.