Add a short description of high-level types

doc/userdoc.org

@@ -1607,6 +1607,46 @@ the output is
 
 *** High-level types<<high-level-types>>
 
+Aside from the basic type described in the previous section, ~pugs~ also
+deals with "high-level" types. This term is more to understand as
+"non-basic types". The ~pugs~ language is not object oriented to keep it
+simple.
+
+To fix ideas let use give a few high-level type of data. These can be
+for instance meshes (the ~mesh~ type), output streams (the ~ostream~
+type), boundary descriptors (~boundary~), quadrature formula
+(~quadrature~), discrete functions (~Vh~),...
+
+One can already see that the complexity of these types may vary a lot.
+
+The main difference between these types and basic types is that,
+high-level types are not available in directly the language but they
+are loaded on demand (using the ~import~ keyword in the preamble of the
+script).
+
+The second difference is that data of these type are *constant*. More
+precisely, the content of high-level variables can be replaced by a
+new one *but* it cannot be modified. For this reason, the following
+operators can never be applied to variables of this kind
+| forbidden operators | description                 |
+|---------------------+-----------------------------|
+| ~++~                  | pre/post increment operator |
+| ~--~                  | pre/post decrement operator |
+| ~+=~                  | assignment by sum           |
+| ~-=~                  | assignment by difference    |
+| ~*=~                  | assignment by product       |
+| ~/=~                  | assignment by quotient      |
+
+We conclude by stating that if access operator ~[]~ can eventually be
+overloaded for high-level types, it is quite difficult and should be
+done with care. It is not recommended.
+
+Finally, the last difference lies in the fact that high-level types
+use shallow copy and not value copy as it is the case for basic
+types. This is transparent to the user and provides the intuitive
+(similar) behavior since data of high-level variables are constant. To
+illustrate this, let us consider the following example.
+
 *** TODO Lists
 
 *** TODO Tuples types