Coding-Conventions.md

-# Coding Conventions
+# Coding Conventions
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+
+## Do not use _Class Instance Variables_
+
+They are not doing what one would expect and they violate the [Liskov Principle](https://en.wikipedia.org/wiki/Liskov_substitution_principle)
+
+Example:
+
+```ruby
+[136] pry(File):1> class A
+[136] pry(File):1*   cattr_accessor :a 
+[136] pry(File):1* end  
+=> [:a]
+[137] pry(File):1> class B < A
+[137] pry(File):1*   cattr_accessor :a
+[137] pry(File):1* end  
+=> [:a]
+[138] pry(File):1> class C < A
+[138] pry(File):1*   cattr_accessor :a
+[138] pry(File):1* end  
+=> [:a]
+[139] pry(File):1> C.a
+=> nil
+[140] pry(File):1> A.a = 42
+=> 42
+[141] pry(File):1> C.a
+=> 42
+[142] pry(File):1> A.a
+=> 42
+[143] pry(File):1> B.a
+=> 42
+[144] pry(File):1> B.a = 84
+=> 84
+[145] pry(File):1> A.a
+=> 84
+[146] pry(File):1> C.a
+=> 84
+[147] pry(File):1>
+
+      
+```
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