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diff --git a/42sh/src/ast/ast_accessors.h b/42sh/src/ast/ast_accessors.h
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+++ b/42sh/src/ast/ast_accessors.h
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+#ifndef AST_ACCESSORS_H
+#define AST_ACCESSORS_H
+
+#include <stddef.h>
+
+#include "ast/ast.h"
+
+/**
+ * @brief Returns the i-th child of `ast`.
+ * The children are stored as follows:
+ * `index` 0: left child, or expression in unary trees, or `condition` in `if`
+ * blocks. `index` 1: right child, or `then` expression in `if` blocks. `index`
+ * 2+: middle children, or `else` expression in `if` blocks (only `index` 2).
+ * @param ast Any kind of AST.
+ * @param index The index of the child to get.
+ * @note All kinds of ASTs can be passed to this function.
+ * All `index` can be passed to this function too.
+ * However, if the given type does not have any child or if the `index` is too
+ * big, `get_i` returns `NULL.`
+ */
+struct ast *get_i(struct ast *ast, size_t index);
+
+/**
+ * @brief Alias for `get_i(ast, 0)`.
+ * @note See `get_i` for a full explanation.
+ */
+struct ast *get_left(struct ast *ast);
+
+/**
+ * @brief Alias for `get_i(ast, 1)`.
+ * @note See `get_i` for a full explanation.
+ */
+struct ast *get_right(struct ast *ast);
+
+/**
+ * @brief Sets the `child` as the i-th child of `ast`.
+ * The children are stored as follows:
+ * `index` 0: left child, or expression in unary trees, or `condition` in `if`
+ * blocks. `index` 1: right child, or `then` expression in `if` blocks. `index`
+ * 2+: middle children, or `else` expression in `if` blocks (only `index` 2).
+ * @param ast Any kind of AST.
+ * @param child The child to be assigned in `ast`.
+ * @param index The position of the child in the `ast`.
+ * @note All kinds of ASTs can be passed to this function.
+ * All `index` can be passed to this function too.
+ * However, if the given type can not contain any child or if the `index` is too
+ * big, `set_i` does nothing.
+ */
+void set_i(struct ast *ast, struct ast *child, size_t index);
+
+/**
+ * @brief Alias for `set_i(ast, child, 0)`.
+ * @note See `set_i` for a full explanation.
+ */
+void set_left(struct ast *ast, struct ast *child);
+
+/**
+ * @brief Alias for `set_i(ast, child, 1)`.
+ * @note See `set_i` for a full explanation.
+ */
+void set_right(struct ast *ast, struct ast *child);
+
+#endif /* ! AST_ACCESSORS_H */