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2.4  INPUT-OUTPUT

	The Ada language is defined in such a way that all input and
output is performed in terms of other language features.  There are
no special intrinsic features just for input and output.  In fact
input-output is just a service required by a program and so is
provided by one or more Ada packages.  This approach runs the
attendant risk that different implementations will provide different
packages and program portability will be compromised.  In order to
avoid this, the Language Reference Manual describes certain standard
packages that can be expected to be available.  Other, more
elaborate, packages may be appropriate to special circumstances and
the language does not prevent this.  Indeed very simple packages such
as our purely illustrative SIMPLE_IO may also be appropriate. 
Further consideration of input and output is deferred until Chapter
15 when we discuss interfaces between our program and the outside
world in general.