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Notation Used in This Chapter

(The following information augments or overrides the information in Section 1.5 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 1 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

In this chapter, "must" denotes a requirement, "may" denotes permission, and "must not" and "may not" denote prohibition. Terms such as "might", "should", and "can" generally add little or nothing in the way of weight to the GNU Fortran language itself, but are used to explain or illustrate the language.

For example:

     ``The FROBNITZ statement must precede all executable
     statements in a program unit, and may not specify any dummy
     arguments.  It may specify local or common variables and arrays.
     Its use should be limited to portions of the program designed to
     be non-portable and system-specific, because it might cause the
     containing program unit to behave quite differently on different

Insofar as the GNU Fortran language is specified, the requirements and permissions denoted by the above sample statement are limited to the placement of the statement and the kinds of things it may specify. The rest of the statement--the content regarding non-portable portions of the program and the differing behavior of program units containing the FROBNITZ statement--does not pertain the GNU Fortran language itself. That content offers advice and warnings about the FROBNITZ statement.

Remember: The GNU Fortran language definition specifies both what constitutes a valid GNU Fortran program and how, given such a program, a valid GNU Fortran implementation is to interpret that program.

It is not incumbent upon a valid GNU Fortran implementation to behave in any particular way, any consistent way, or any predictable way when it is asked to interpret input that is not a valid GNU Fortran program.

Such input is said to have undefined behavior when interpreted by a valid GNU Fortran implementation, though an implementation may choose to specify behaviors for some cases of inputs that are not valid GNU Fortran programs.

Other notation used herein is that of the GNU texinfo format, which is used to generate printed hardcopy, on-line hypertext (Info), and on-line HTML versions, all from a single source document. This notation is used as follows: