diff --git a/setup.py b/setup.py
index c6c9d6f..023fdc0 100644
--- a/setup.py
+++ b/setup.py
@@ -13,7 +13,7 @@ def read(name):
 
 setup(
     name="arpa",
-    version="0.1.0a2",
+    version="0.1.0a3",
     author="Stefan Fischer",
     author_email="sfischer13@ymail.com",
     url="https://github.com/sfischer13/python-arpa/",
@@ -21,13 +21,15 @@ def read(name):
     description="This is a library for reading ARPA n-gram models.",
     long_description=read("README.rst"),
     keywords="ARPA n-gram ngram language model LM language technology LT "
-    "natural language processing NLP",
+    "computational linguistics CL natural language processing NLP",
     install_requires=["enum34"],
     packages=find_packages("src"),
     package_dir={"": "src"},
     classifiers=[
         "Development Status :: 4 - Beta",
         "Intended Audience :: Developers",
+        "Intended Audience :: Information Technology",
+        "Intended Audience :: Science/Research",
         "License :: OSI Approved :: MIT License",
         "Operating System :: OS Independent",
         "Programming Language :: Python",
@@ -36,6 +38,8 @@ def read(name):
         "Programming Language :: Python :: 3.2",
         "Programming Language :: Python :: 3.3",
         "Programming Language :: Python :: 3.4",
+        "Topic :: Scientific/Engineering",
+        "Topic :: Scientific/Engineering :: Artificial Intelligence",
         "Topic :: Software Development :: Libraries",
         "Topic :: Software Development :: Libraries :: Python Modules",
         "Topic :: Text Processing",
diff --git a/src/arpa/__init__.py b/src/arpa/__init__.py
index 113a593..207b1da 100644
--- a/src/arpa/__init__.py
+++ b/src/arpa/__init__.py
@@ -22,10 +22,15 @@
 """
 This is a library for reading ARPA n-gram models.
 
-It may be imported directly:
+The package may be imported directly::
 
     import arpa
 
+Details about the ARPA n-gram format can be found here:
+
+- `SRILM <http://www.speech.sri.com/projects/srilm/manpages/ngram-format.5.html>`_
+- `ICSI Speech <http://www1.icsi.berkeley.edu/Speech/faq/grammarfmts.html>`_
+
 The library was initiated by Stefan Fischer and is developed and maintained by many others.
 """
 
@@ -40,4 +45,4 @@
 __date__ = "2015-07-05"
 __license__ = "MIT"
 __status__ = "development"
-__version__ = "0.1.0a2"
+__version__ = "0.1.0a3"
diff --git a/src/arpa/arpa.py b/src/arpa/arpa.py
index d754c10..38776f5 100644
--- a/src/arpa/arpa.py
+++ b/src/arpa/arpa.py
@@ -5,22 +5,26 @@
 
 
 def dump(obj, fp):
+    """Serialize obj to fp (a file-like object) in ARPA format."""
     raise NotImplementedError
     # TODO: obj.write(fp)
 
 
 def dumpf(obj, path, mode="wt", encoding=None):
+    """Serialize obj to path in ARPA format."""
     with open(path, mode=mode, encoding=encoding) as f:
         dump(obj, f)
 
 
 def dumps(obj):
+    """Serialize obj to an ARPA formatted str."""
     with StringIO() as f:
         dump(obj, f)
         return f.getvalue()
 
 
 def load(fp, model=None, parser=None):
+    """Deserialize fp (a file-like object) to a Python object."""
     if not model:
         model = "simple"
     if not parser:
@@ -38,10 +42,12 @@ def load(fp, model=None, parser=None):
 
 
 def loadf(path, mode="rt", encoding=None, model=None, parser=None):
+    """Deserialize path (a text file) to a Python object."""
     with open(path, mode=mode, encoding=encoding) as f:
         return load(f, model, parser)
 
 
 def loads(s, model=None, parser=None):
+    """Deserialize s (a str) to a Python object."""
     with StringIO(s) as f:
         return load(f, model, parser)
diff --git a/src/arpa/exceptions.py b/src/arpa/exceptions.py
index c393e56..15a0c1d 100644
--- a/src/arpa/exceptions.py
+++ b/src/arpa/exceptions.py
@@ -2,9 +2,10 @@
 
 
 class ARPAException(Exception):
-    """Base exception that is never raised."""
+    """Common base class for all package exceptions."""
     pass
 
 
 class ParseException(ARPAException):
+    """ARPA file could not be parsed."""
     pass
diff --git a/src/arpa/models/base.py b/src/arpa/models/base.py
index ae29dc1..0f9602b 100644
--- a/src/arpa/models/base.py
+++ b/src/arpa/models/base.py
@@ -15,15 +15,7 @@ def add_entry(self, ngram, p, bo=None, order=None):
         pass
 
     def log10_p(self, ngram):
-        if not ngram:
-            raise ValueError
-        elif isinstance(ngram, str):
-            ngram = self._str2tuple(ngram)
-        elif isinstance(ngram, list):
-            ngram = tuple(ngram)
-        elif not isinstance(ngram, tuple):
-            raise ValueError
-
+        ngram = self._check_input(ngram)
         try:
             return self._log10_p(ngram)
         except KeyError:
@@ -33,9 +25,16 @@ def log10_p(self, ngram):
                 log10_bo = 0
             return log10_bo + self.log10_p(ngram[1:])
 
+    def log10_s(self, sentence):
+        words = self._check_input(sentence)
+        return sum(self.log10_p(words[:i]) for i in range(1, len(words) + 1))
+
     def p(self, ngram):
         return 10 ** self.log10_p(ngram)
 
+    def s(self, sentence):
+        return 10 ** self.log10_s(sentence)
+
     @abstractmethod
     def _log10_bo(self, ngram):
         pass
@@ -45,5 +44,14 @@ def _log10_p(self, ngram):
         pass
 
     @staticmethod
-    def _str2tuple(s):
-        return tuple(s.strip().split(" "))
+    def _check_input(input):
+        if not input:
+            raise ValueError
+        elif isinstance(input, tuple):
+            return input
+        elif isinstance(input, list):
+            return tuple(input)
+        elif isinstance(input, str):
+            return tuple(input.strip().split(" "))
+        else:
+            raise ValueError