Added section Avoid obsolete language elements (#31)

* Added section How to Check Automatically

* Explain example a little better

* Updated the cheat sheet to 1.3.0

Added section Avoid obsolete language elements

clean-abap/CleanABAP.md

@@ -32,6 +32,7 @@ The [Cheat Sheet](cheat-sheet/CheatSheet.md) is a print-optimized version.
   - [Mind the performance](#mind-the-performance)
   - [Prefer object orientation to procedural programming](#prefer-object-orientation-to-procedural-programming)
   - [Prefer functional to procedural language constructs](#prefer-functional-to-procedural-language-constructs)
+  - [Avoid obsolete language elements](#avoid-obsolete-language-elements)
   - [Use design patterns wisely](#use-design-patterns-wisely)
 - [Constants](#constants)
   - [Use constants instead of magic numbers](#use-constants-instead-of-magic-numbers)
@@ -591,6 +592,54 @@ IF line_exists( value_pairs[ name = 'A' ] ).
 
 Many of the detailed rules below are just specific reiterations of this general advice.
 
+### Avoid obsolete language elements
+
+> [Clean ABAP](#clean-abap) > [Content](#content) > [Language](#language) > [This section](#avoid-obsolete-language-elements)
+
+When upgrading your ABAP version,
+make sure to check for obsolete language elements
+and refrain from using them.
+
+For example, the `@`-escaped "host" variables
+in the following statement make a little clearer
+what's a program variable and what's a column in the database,
+
+```ABAP
+SELECT *
+  FROM spfli
+  WHERE carrid = @carrid AND
+        connid = @connid
+  INTO TABLE @itab.
+```
+
+as compared to the [obsolete unescaped form](https://help.sap.com/doc/abapdocu_750_index_htm/7.50/en-US/abenopen_sql_hostvar_obsolete.htm)
+
+```ABAP
+SELECT *
+  FROM spfli
+  WHERE carrid = carrid AND
+        connid = connid
+  INTO TABLE itab.
+```
+
+Newer alternatives tend to improve readability of the code,
+and reduce design conflicts with modern programming paradigms,
+such that switching to them can automatically clean your code.
+
+While continuing to work, obsolete elements may stop benefitting
+from optimizations in terms of processing speed and memory consumption.
+
+With modern language elements, you can onboard young ABAPers easier,
+who may no longer be familiar with the outdated constructs
+because they are no longer taught in SAP's trainings.
+
+The SAP NetWeaver documentation contains a stable section
+that lists obsolete language elements, for example
+[NW 7.50](https://help.sap.com/doc/abapdocu_750_index_htm/7.50/en-US/index.htm?file=abenabap_obsolete.htm),
+[NW 7.51](https://help.sap.com/doc/abapdocu_751_index_htm/7.51/en-US/index.htm?file=abenabap_obsolete.htm),
+[NW 7.52](https://help.sap.com/doc/abapdocu_752_index_htm/7.52/en-US/index.htm?file=abenabap_obsolete.htm),
+[NW 7.53](https://help.sap.com/doc/abapdocu_753_index_htm/7.53/en-US/index.htm?file=abenabap_obsolete.htm).
+
 ### Use design patterns wisely
 
 > [Clean ABAP](#clean-abap) > [Content](#content) > [Language](#language) > [This section](#use-design-patterns-wisely)

clean-abap/cheat-sheet/CheatSheet.md

@@ -12,14 +12,16 @@ is versioned according to the [Semantic Versioning specification](https://semver
 
 ## Download
 
-- [**Cheat Sheet**](CleanABAPCheatSheetV1.2.1.pdf) · Latest version · PDF
+- [**Cheat Sheet**](CleanABAPCheatSheetV1.3.0.pdf) · Latest version · PDF
 
 - [**The Golden Rules**](CleanABAPTheGoldenRulesV1.0.0.pdf) · Latest version · PDF
 
 ## Versions
 
 ### Cheat Sheet
 
+- [v1.3.0](CleanABAPCheatSheetV1.3.0.pdf) · 8 May 2019
+  - Added section _Avoid obsolete language elements_
 - [v1.2.1](CleanABAPCheatSheetV1.2.1.pdf) · 7 May 2019
   - Replaced _imperative programming_ with _procedural programming_
 - [v1.2.0](CleanABAPCheatSheetV1.2.0.pdf) · 6 May 2019