diff --git a/openmc/data/function.py b/openmc/data/function.py
index e87f0a9c42..c8fa15b768 100644
--- a/openmc/data/function.py
+++ b/openmc/data/function.py
@@ -410,7 +410,9 @@ class Combination(EqualityMixin):
         Functions which are to be added together
     operations : Iterable of numpy.ufunc
         Operations to perform between functions; note that the standard order
-        of operations (i.e., PEMDAS) will not be followed.
+        of operations will not be followed, but can be simulated by
+        combinations of Combination objects. The operations parameter must have
+        a length one less than the number of functions.
 
 
     Attributes
@@ -427,9 +429,9 @@ class Combination(EqualityMixin):
         self.operations = operations
 
     def __call__(self, x):
-        ans = np.zeros_like(x)
+        ans = self.functions[0](x)
         for i, operation in enumerate(self.operations):
-            ans = operation(ans, self.functions[i](x))
+            ans = operation(ans, self.functions[i + 1](x))
         return ans
 
     @property
@@ -448,7 +450,7 @@ class Combination(EqualityMixin):
     @operations.setter
     def operations(self, operations):
         cv.check_type('operations', operations, Iterable, np.ufunc)
-        length = len(self.functions)
+        length = len(self.functions) - 1
         cv.check_length('operations', operations, length, length_max=length)
         self._operations = operations
 
@@ -488,7 +490,7 @@ 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 +501,9 @@ 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()