BNF is one of the two main notation techniques for context-free grammars.

Example:

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A sandwich consists of a lower slice of bread, mustard or mayonnaise;
optional lettuce, an optional slice of tomato; two to four slices of
either bologna, salami, or ham (in any combination); one or more slices
of cheese, and a top slice of bread.

This translates to:

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sandwich ::= lower_slice [mustard|mayonnaise] lettuce? tomato?
             [bologna|salami|ham]{2,4} cheese+ top_slice

References: AListApart, Wikipedia

Bundler accepts config options per gem, eg:

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bundle config build.qrencoder \
  --with-qrencoder-include=/usr/local/include/qrencode.h
  --with-qrencoder-lib=/usr/local/lib/libqrencode.dylib 

1. Before you begin, place the tip of your tongue on the roof of your mouth just above your teeth and keep it there throughout the exercise.
2. Exhale completely through your mouth quite forcefully so you make a whoosh sound.
3. Close your mouth and inhale quietly and softly through your nose for a mental count of four.
4. Hold your breath and count to seven.
5. Next, exhale completely through your mouth, making another whoosh sound for eight seconds in one large breath.
6. Now inhale again and repeat the cycle three times for a total of four breaths.

All inhaling breaths must be quiet and through your nose and all exhaling breaths must be loud and through your mouth.

Source: Distractify.com