[FIX]: Gazprea iterators are non-mutable within their nested scope

gazprea/spec/expressions.rst

@@ -144,8 +144,11 @@ instance:
 
 This is true for domain expressions within generators as well.
 
-Iterator variables can be assigned to and re-declared within the enclosed iterator loop.
-The variable is re-initialized according to the expression each iteration.
+An iterator variable is a fresh *constant* binding in each iteration:
+assigning to it is an error, but a declaration inside the loop body may
+shadow it, exactly as in any other nested scope. The shadowing declaration
+does not affect the iteration; the iterator is re-bound from the domain at
+the start of each iteration.
 
 ::
 

gazprea/spec/statements.rst

@@ -359,8 +359,10 @@ Array ranges can also be used instead:
              i -> std_output;
            }
 
-The domain is evaluated once during the first iteration of the loop. Each iteration
-defines a constant domain variable from it's respective index. For instance:
+The domain is evaluated once during the first iteration of the loop. Each
+iteration defines a constant domain variable from its respective index;
+the variable cannot be assigned to, though it may be shadowed by an inner
+declaration (see :ref:`ssec:expressions_dom_expr`). For instance:
 
 ::