too much indent

lib/Text/Indented.pm

@@ -1,3 +1,24 @@
 grammar Text::Indented;
 
-regex TOP { .* }
+class TooMuchIndent is Exception {}
+
+constant TABSTOP = 4;
+
+regex TOP {
+    :my @*SUITES = "root";
+
+    <line>*
+}
+
+sub indent { @*SUITES.end }
+
+regex line {
+    ^^ (<{ "\\x20" x TABSTOP }>*) (\h*) (\N*) $$ \n?
+
+    {
+        my $new_indent = $0.chars div TABSTOP;
+
+        die TooMuchIndent.new
+            if $new_indent > indent() + 1;
+    }
+}

t/parse-indent.t

@@ -41,4 +41,13 @@ sub fails_with($input, $ex_type, $message = $ex_type.^name) {
     parses_correctly($input, 'single indent');
 }
 
+{
+    my $input = q:to/EOF/;
+    Level 1
+            Level 3!
+    EOF
+
+    fails_with($input, Text::Indented::TooMuchIndent);
+}
+
 done;