Document the listing module.

README.md

@@ -80,6 +80,9 @@ Functional Modules
 Support Modules
 ---------------
 
+There modules are not specific to this project but had to be
+implemented anyway to not require any external dependencies.
+
 ### ANSI
 
 ### Command Line parsing
@@ -88,6 +91,43 @@ Support Modules
 
 ### Listing
 
+This module implements listing a text file with line numbers and
+optional highlights of given source lines, optionally with comments
+for those lines.
+
+    (print-source-listing lines highlights context hl-pre hl-post ctx-pre ctx-post ellipsis)
+
+* ```lines``` - list of string representing lines of the text file
+* ```highlights``` - list of highlights (see below)
+* ```context``` - number of context lines to be shown around highlighted lies
+* ```hl-pre``` - string introducing highlighted lines
+* ```hl-post``` - string terminating highlighted lines
+* ```ctx-pre``` - string introducing context lines
+* ```ctx-post``` - string terminating context lines
+* ```ellipsis``` - string representing lines omitted from the output
+
+Prints given text file represented by the ```lines``` list of strings.
+
+Lines to be highlighted can be specified in the ```highlights```
+list. The highlight specification is either a number or a list
+containing the line number and string comment.
+
+The lines are actually highlighted by prepending them with
+```hl-pre``` string and the highlight is finished by appending
+```hl-post``` to them. Usually some constants from the ```ansi```
+module are used.
+
+If some lines are highlighted a number of ```context``` lines
+surrounding them may be printed as well. If this argument is negative,
+all non-highlighted lines are printed as context lines.
+
+Context lines are prepended with ```ctx-pre``` string and terminated
+by ```ctx-post``` string.
+
+If some lines between highlight and/or context lines are omitted,
+```ellipsis``` string is printed on single line as a substitute.
+
+
 ### Progress
 
 Provides syntax forms and procedures for showing progress of a