Write first draft of Bison-based parser and lexer

content/blog/02_compiler_parsing.md

@@ -226,6 +226,10 @@ It's called Bison, and it is frequently used with Flex. Before we get to bison,
 incurred - the implementation of our AST. Such a tree is language-specific, so Bison doesn't generate it for us. Here's what
 I came up with:
 {{< codeblock "C++" "compiler_ast.hpp" >}}
+We create a base class for an expression tree, which we call `ast`. Then, for each possible syntactic construct in our language
+(a number, a variable, a binary operation, a case expression) we create a subclass of `ast`. The `ast_case` subclass
+is the most complex, since it must contain a list of case expression branches, which are a combination of a `pattern` and
+another expression.
 
 Finally, we get to writing our Bison file, `parser.y`. Here's what I come up with:
 {{< rawblock "compiler_parser.y" >}}