diff --git a/docs/source/code.rst b/docs/source/code.rst
index fc4ba5071ebccb01008e2ea1ea75ac8d473c39a2..80c1e348ee6fa314530cbd2bac9d8da8192cd5a6 100644
--- a/docs/source/code.rst
+++ b/docs/source/code.rst
@@ -142,12 +142,11 @@ There is also a set of convenience functions to generate fields with constant
 values or fields filled with random numbers according to a user-specified
 distribution.
 
-Fields are the only fundamental NIFTy objects which can change state after they
-have been constructed: while their data type, domain, and array shape cannot
-be modified, the actual data content of the array may be manipulated during the
-lifetime of the object. This is a slight deviation from the philosophy that all
-NIFTy objects should be immutable, but this choice offers considerable
-performance benefits.
+Like almost all NIFTy objects, fields are immutable: their value or any other
+attribute cannot be modified after construction. To manipulate a field in ways
+that are not covered by the provided standard operations, its data content must
+be extracted first, then changed, and a new field has to be created from the
+result.
 
 
 Linear Operators
diff --git a/nifty5/__init__.py b/nifty5/__init__.py
index 04e19aa4e6048175661d39a654418c4b2a99150f..5a2c94c78cf5292fac3f696ae8a2619910863859 100644
--- a/nifty5/__init__.py
+++ b/nifty5/__init__.py
@@ -89,7 +89,7 @@ from .library.bernoulli_energy import BernoulliEnergy
 
 from . import extra
 
-from .utilities import memo
+from .utilities import memo, frozendict
 
 from .logger import logger