diff --git a/tldp/sources.py b/tldp/sources.py
index eab2e1a..6633a46 100644
--- a/tldp/sources.py
+++ b/tldp/sources.py
@@ -12,11 +12,47 @@ from .typeguesser import guess, knownextensions
 
 
 class SourceCollection(collections.MutableMapping):
+    '''a dict-like container for SourceDocument objects
 
+    The key in the SourceCollection is the stem name of the document, which
+    allows convenient access and guarantees non-collision.
+
+    The use of the stem as a key works conveniently with the
+    OutputCollection which uses the same strategy on OutputDirectory.
+    '''
     def __repr__(self):
         return '<%s:(%s docs)>' % (self.__class__.__name__, len(self))
 
     def __init__(self, args=None):
+        '''construct a SourceCollection
+
+        If args is not supplied, SourceCollection is basically a dict().
+        If args is supplied, then SourceCollection ensures it is operating on
+        the absolute filesystem path for each of the source directories.
+
+        If any of the supplied args does not exist as a directory,
+        SourceCollection will log the missing source directory names and then
+        will raise an IOError and quit.
+
+        For each document that it finds in a source directory, it creates a
+        SourceDocument entry in itself using the stem name as a key.
+
+        The rules for identifying possible SourceDocuments go as follows.
+        Within any source directory, a source document can consist of a single
+        file with an extension or a directory.
+
+        If the candidate entry is a directory, then, the stem is the full
+        directory name, e.g. Masquerading-Simple-HOWTO
+
+        If the candidate entry is a file, the stem is the filename minus
+        extension, e.g. Encrypted-Root-Filesystem-HOWTO
+
+        Because the SourceCollection accepts (and will scan) many source
+        directories, it is possible that there will be stem name collisions.
+        If it discovers a stem collision, SourceCollection will issue a
+        warning and skip the duplicated stem(s).  [It also tries to process
+        the source directories and candidates in a stable order between runs.]
+        '''
         if args is None:
             return
         dirs = [os.path.abspath(x) for x in args]
@@ -70,13 +106,39 @@ class SourceCollection(collections.MutableMapping):
 
 
 class SourceDocument(object):
-
+    '''a class providing a container for each set of source documents
+    '''
     def __repr__(self):
         return '<%s:%s (%s)>' % \
                (self.__class__.__name__, self.filename, self.doctype)
 
     def __init__(self, filename):
-        # -- canonicalize the pathname we are given.
+        '''construct a SourceDocument
+
+        filename is a required parameter
+
+        The filename is the main (and sometimes sole) document representing
+        the source of the LDP HOWTO or Guide.  It is the document that is
+        passed by name to be handled by any document processing toolchains
+        (see also tldp.doctypes).
+
+        Each instantiation will raise an IOERror if the supplied filename does
+        not exist or if the filename isn't a file (symlink is fine, directory
+        or fifo is not).
+
+        The remainder of the instantiation will set attributes that are useful
+        later in the processing phase, for example, stem, status, enclosing
+        directory name and file extension.
+
+        There are two important attributes.  First, the document type guesser
+        will try to infer the doctype (from file extension and signature).
+        Note that it is not a fatal error if document type cannot be guessed,
+        but the document will not be able to be processed.  Second, it is
+        useful during the decision-making process to know if any of the source
+        files are newer than the output files. Thus, the stat() information
+        for every file in the source document directory (or just the single
+        source document file) will be collected.
+        '''
         self.filename = os.path.abspath(filename)
         if not os.path.exists(self.filename):
             logger.critical("Missing source document: %s", self.filename)
@@ -85,7 +147,7 @@ class SourceDocument(object):
             logger.critical("Source document is not a plain file: %s", self.filename)
             raise TypeError("Wrong type, not a plain file: " + self.filename)
 
-        self.doctype = self._doctype()
+        self.doctype = guess(self.filename)
         self.status = 'source'
         self.dirname, self.basename = os.path.split(self.filename)
         self.stem, self.ext = os.path.splitext(self.basename)
@@ -96,8 +158,5 @@ class SourceDocument(object):
         else:
             self.statinfo = statfiles(self.filename, relative=self.dirname)
 
-    def _doctype(self):
-        return guess(self.filename)
-
 #
 # -- end of file