more updates from master

doc/tutorial/gf-tutorial.t2t

@@ -1188,7 +1188,7 @@ use ``generate_trees = gt``.
   this wine is fresh
   this wine is warm
 ```
-The default **depth** is 3; the depth can be
+The default **depth** is 5; the depth can be
 set by using the ``depth`` flag:
 ```
   > generate_trees -depth=2 | l
@@ -1265,10 +1265,16 @@ Human eye may prefer to see a visualization: ``visualize_tree = vt``:
   > parse "this delicious cheese is very Italian" | visualize_tree
 ```
 The tree is generated in postscript (``.ps``) file. The ``-view`` option is used for
-telling what command to use to view the file. Its default is ``"open"``, which works
-on Mac OS X. On Ubuntu Linux, one can write
+telling what command to use to view the file.
+
+This works on Mac OS X:
 ```
-  > parse "this delicious cheese is very Italian" | visualize_tree -view="eog"
+  > parse "this delicious cheese is very Italian" | visualize_tree -view=open
+```
+On Linux, one can use one of the following commands.
+```
+  > parse "this delicious cheese is very Italian" | visualize_tree -view=eog
+  > parse "this delicious cheese is very Italian" | visualize_tree -view=xdg-open
 ```
 
 
@@ -1733,6 +1739,13 @@ A new module can **extend** an old one:
       Pizza : Kind ;
   }
 ```
+Note that the extended grammar doesn't inherit the start
+category from the grammar it extends, so if you want to
+generate sentences with this grammar, you'll have to either
+add a startcat (e.g. ``flags startcat = Question ;``),
+or in the GF shell, specify the category to ``generate_random`` or ``geneate_trees``
+(e.g. ``gr -cat=Comment`` or ``gt -cat=Question``).
+
 Parallel to the abstract syntax, extensions can
 be built for concrete syntaxes:
 ```
@@ -3733,7 +3746,7 @@ However, type-incorrect commands are rejected by the typecheck:
   The parsing is successful but the type checking failed with error(s):
     Couldn't match expected type Device light
        against the interred type Device fan
-    In the expression: DKindOne fan       
+    In the expression: DKindOne fan
 ```
 
 #NEW
@@ -4171,7 +4184,7 @@ division of integers.
 ```
   abstract Calculator = {
   flags startcat = Exp ;
-  
+
   cat Exp ;
 
   fun
@@ -4578,7 +4591,7 @@ in any multilingual grammar between any languages in the grammar.
 module Main where
 
 import PGF
-import System (getArgs)
+import System.Environment (getArgs)
 
 main :: IO ()
 main = do