diff --git a/openmc/data/function.py b/openmc/data/function.py
index e87f0a9c42..4a1f9d7a38 100644
--- a/openmc/data/function.py
+++ b/openmc/data/function.py
@@ -428,8 +428,8 @@ class Combination(EqualityMixin):
 
     def __call__(self, x):
         ans = np.zeros_like(x)
-        for i, operation in enumerate(self.operations):
-            ans = operation(ans, self.functions[i](x))
+        for op, func in zip(self.operations, self.functions):
+            ans = op(ans, func(x))
         return ans
 
     @property
@@ -488,7 +488,7 @@ class Sum(EqualityMixin):
         self._functions = functions
 
 
-class Piecewise(EqualityMixin):
+class Regions1D(EqualityMixin):
     """Piecewise composition of multiple functions.
 
     This class allows you to create a callable object which is composed
@@ -499,9 +499,9 @@ class Piecewise(EqualityMixin):
     functions : Iterable of Callable
         Functions which are to be combined in a piecewise fashion
     breakpoints : Iterable of float
-        The breakpoints between each function. The functions
-        in the functions attribute must be provided in order of
-        increasing intervals.
+        The values of the dependent variable that define the domain of
+        each function. The *i*th and *(i+1)*th values are the limits of the
+        domain of the *i*th function. Values must be monotonically increasing.
 
     Attributes
     ----------
diff --git a/openmc/data/library.py b/openmc/data/library.py
index 3e01c24643..0363de4fc5 100644
--- a/openmc/data/library.py
+++ b/openmc/data/library.py
@@ -22,9 +22,19 @@ class DataLibrary(EqualityMixin):
     def __init__(self):
         self.libraries = []
 
-    def __getitem__(self, material):
+    def get_by_materials(self, value):
+        """Access a library entry by passing a value of the 'materials'
+        attribute of the library entry.
+
+        Returns
+        -------
+        library : dict
+            Dictionary summarizing cross section data from a single file;
+            the dictionary has keys 'path', 'type', and 'materials'.
+
+        """
         for library in self.libraries:
-            if material in library['materials']:
+            if value in library['materials']:
                 return library
         return None
 
@@ -94,6 +104,11 @@ class DataLibrary(EqualityMixin):
         path : str
             Path to XML file to read.
 
+        Returns
+        -------
+        data : openmc.data.DataLibrary
+            openmc.data.DataLibrary object initialized from the provided XML
+
         """
 
         data = cls()