( Pattern Intro

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 10.5.1 Introduction to Pattern Rules
 A pattern rule contains the character `%' (exactly one of them) in the
 target; otherwise, it looks exactly like an ordinary rule.  The target
 is a pattern for matching file names; the `%' matches any nonempty
 substring, while other characters match only themselves.  
    For example, `%.c' as a pattern matches any file name that ends in
 `.c'.  `s.%.c' as a pattern matches any file name that starts with
 `s.', ends in `.c' and is at least five characters long.  (There must
 be at least one character to match the `%'.)  The substring that the
 `%' matches is called the "stem".
    `%' in a prerequisite of a pattern rule stands for the same stem
 that was matched by the `%' in the target.  In order for the pattern
 rule to apply, its target pattern must match the file name under
 consideration and all of its prerequisites (after pattern substitution)
 must name files that exist or can be made.  These files become
 prerequisites of the target.  
    Thus, a rule of the form
      %.o : %.c ; COMMAND...
 specifies how to make a file `N.o', with another file `N.c' as its
 prerequisite, provided that `N.c' exists or can be made.
    There may also be prerequisites that do not use `%'; such a
 prerequisite attaches to every file made by this pattern rule.  These
 unvarying prerequisites are useful occasionally.
    A pattern rule need not have any prerequisites that contain `%', or
 in fact any prerequisites at all.  Such a rule is effectively a general
 wildcard.  It provides a way to make any file that matches the target
 pattern.   Last Resort.
    Pattern rules may have more than one target.  Unlike normal rules,
 this does not act as many different rules with the same prerequisites
 and commands.  If a pattern rule has multiple targets, `make' knows that
 the rule's commands are responsible for making all of the targets.  The
 commands are executed only once to make all the targets.  When searching
 for a pattern rule to match a target, the target patterns of a rule
 other than the one that matches the target in need of a rule are
 incidental: `make' worries only about giving commands and prerequisites
 to the file presently in question.  However, when this file's commands
 are run, the other targets are marked as having been updated themselves.  
    The order in which pattern rules appear in the makefile is important
 since this is the order in which they are considered.  Of equally
 applicable rules, only the first one found is used.  The rules you
 write take precedence over those that are built in.  Note however, that
 a rule whose prerequisites actually exist or are mentioned always takes
 priority over a rule with prerequisites that must be made by chaining
 other implicit rules.  
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