Production  = production_name "=" [ Expression ] "." .
Expression  = Term { "|" Term } .
Term        = Factor { Factor } .
Factor      = production_name | token [ "…" token ] | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .
</pre>

<p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>

pkg io/fs, method (*PathError) Timeout() bool
pkg io/fs, method (*PathError) Unwrap() error
pkg io/fs, method (FileMode) IsDir() bool
pkg io/fs, method (FileMode) IsRegular() bool
pkg io/fs, method (FileMode) Perm() FileMode
pkg io/fs, method (FileMode) String() string
pkg io/fs, method (FileMode) Type() FileMode
pkg io/fs, type DirEntry interface { Info, IsDir, Name, Type }
pkg io/fs, type DirEntry interface, Info() (FileInfo, error)