2017-10-08: warn about partial unindent stable v0.9.6

warn about partial unindent

diff --git a/NEWS b/NEWS
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,9 @@
 wisp ():
 
+wisp 0.9.6:
+- Add a warning with undefined indentation levels: they can
+  produce surprising behaviour when changing existing code.
+
 wisp 0.9.5:
 - Fix bug with using not yet defined lower indentation levels.
   See tests/partial-indent.{w,scm}
diff --git a/configure.ac b/configure.ac
--- a/configure.ac
+++ b/configure.ac
@@ -1,7 +1,7 @@
 dnl run `autoreconf -i` to generate a configure script. 
 dnl Then run ./configure to generate a Makefile.
 dnl Finally run make to generate the project.
-AC_INIT([wisp], [0.9.5],
+AC_INIT([wisp], [0.9.6],
         [arne_bab@web.de])
 # Add macros in m4/ to ensure that wisp builds without having Guile in the aclocal path
 AC_CONFIG_MACRO_DIR([m4])
diff --git a/docs/tutorial.org b/docs/tutorial.org
new file mode 100644
--- /dev/null
+++ b/docs/tutorial.org
@@ -0,0 +1,1 @@
+- for devs from other languages: the only line terminator is the newline. ; starts a comment till the end of the line.
diff --git a/wisp-scheme.w b/wisp-scheme.w
--- a/wisp-scheme.w
+++ b/wisp-scheme.w
@@ -362,7 +362,6 @@ define : wisp-scheme-read-chunk-lines po
                      . #f ; incomment
                      . currentindent
                      ; this also takes care of the hashbang and leading comments.
-                     ; TODO: If used from Guile, activate curly infix via read-options.
                      append currentsymbols : list : wisp-read port
                      . emptylines
 
@@ -531,12 +530,15 @@ define : wisp-scheme-indentation-to-pare
                        let : : previous-indentation : car : cdr indentation-levels
                          if : <= current-line-indentation previous-indentation
                             values processed unprocessed
-                            loop ;; not yet used level! TODO: maybe throw an error here.
-                              . processed
-                              . unprocessed
-                              cons ; recursion via the indentation-levels
-                                . current-line-indentation 
-                                cdr indentation-levels
+                            begin ;; not yet used level! TODO: maybe throw an error here instead of a warning.
+                                let : : linenumber : - (length lines) (length unprocessed)
+                                    format (current-error-port) ";;; WARNING:~A: used lower but undefined indentation level (line ~A of the current chunk: ~S). This makes refactoring much more error-prone, therefore it might become an error in a later version of Wisp.\n" linenumber linenumber (cdr current-line)
+                                loop
+                                  . processed
+                                  . unprocessed
+                                  cons ; recursion via the indentation-levels
+                                    . current-line-indentation 
+                                    cdr indentation-levels
                      : = current-indentation current-line-indentation
                        ; display "current-indent = next-line\n"
                        let