Add reference+description of PENS scale

cnl.md

@@ -23,7 +23,7 @@ Example: don't allow [PP attachment](https://www.gabormelli.com/RKB/Prepositiona
 
 Variations [man with a telescope] and [saw with a telescope] need to be marked explicitly:
 
-1. I saw the (man with a telescope)  
+1. I saw the (man with a telescope)
 1. I (saw with a telescope) the man
 
 ### Semantic
@@ -58,3 +58,27 @@ The CNL described makes it possible to automatically translate accessibility des
 Example: [Attempto Controlled English](https://en.wikipedia.org/wiki/Attempto_Controlled_English)
 
 A fragment of (semi-natural) English which can be unambiguously translated into first-order logic.
+
+## Types of CNLs
+
+[Kuhn (2014)](https://www.aclweb.org/anthology/J14-1005.pdf) introduces the PENS classification scheme:
+
+<!-- @article{kuhn2014survey, -->
+<!--   title={A survey and classification of controlled natural languages}, -->
+<!--   author={Kuhn, Tobias}, -->
+<!--   journal={Computational linguistics}, -->
+<!--   volume={40}, -->
+<!--   number={1}, -->
+<!--   pages={121--170}, -->
+<!--   year={2014}, -->
+<!--   publisher={MIT Press} -->
+<!-- } -->
+
+* __Precision__: "Ambiguity, predictability, andformality  of  definition"
+* __Expressiveness__: how many different things can you say with the language
+* __Naturalness__: "Grammar modifications, understandability, and natural look-and-feel"
+* __Simplicity__: how is the grammar defined? a simple CFG that fits one page, or no complete description exists?
+
+Quote from the paper:
+
+> PENS  drastically  simplifies  things  by  restricting  each  ofits four dimensions to five classes, to be numbered from 1 to 5. These five classes are non-overlapping and consecutively cover the one-dimensional space between the two extremes: English on the one end and propositional logic on the other. For precision and simplicity, English is on the bottom end of the scale in class 1, which we write as P¹ and S¹. Propositional logic is on the opposite end of the scale in class 5, represented withP5and S5. For expressiveness and naturalness, the roles are switched: English is at the top end (E⁵ and N⁵) and propositional logic at the bottom (E¹ and N¹). In this way, the scheme defines a conceptual space for CNLs that includes natural and formal languagesas  special  cases.  Combining  the  four  dimensions  gives  5⁴=625  classes,  representedwith shorthand such as P¹E⁵N⁵S¹ for English and P⁵E¹N¹S⁵for propositional logic. The difficult and interesting part of this intellectual exercise is where and how to draw the borders between the five classes of each dimension.