Addition for energies

nifty4/library/__init__.py

-from .wiener_filter_energy import WienerFilterEnergy
-from .wiener_filter_curvature import WienerFilterCurvature
+from .los_response import LOSResponse
 from .noise_energy import NoiseEnergy
 from .nonlinear_power_energy import NonlinearPowerEnergy
 from .nonlinear_wiener_filter_energy import NonlinearWienerFilterEnergy
+from .nonlinearities import Exponential, Linear, PositiveTanh, Tanh
 from .poisson_energy import PoissonEnergy
-from .nonlinearities import Exponential, Linear, Tanh, PositiveTanh
-from .los_response import LOSResponse
+from .unit_log_gauss import UnitLogGauss
+from .wiener_filter_curvature import WienerFilterCurvature
+from .wiener_filter_energy import WienerFilterEnergy

nifty4/minimization/energy.py

@@ -16,7 +16,9 @@
 # NIFTy is being developed at the Max-Planck-Institut fuer Astrophysik
 # and financially supported by the Studienstiftung des deutschen Volkes.
 
-from ..utilities import memo, NiftyMetaBase
+from ..field import Field
+from ..multi import MultiField
+from ..utilities import NiftyMetaBase, memo
 
 
 class Energy(NiftyMetaBase()):
@@ -125,3 +127,53 @@ class Energy(NiftyMetaBase()):
             `dir`. If None, the step size is not limited.
         """
         return None
+
+    def __add__(self, other):
+        assert isinstance(other, Energy)
+        return Add(self, other)
+
+    def __sub__(self, other):
+        assert isinstance(other, Energy)
+        return Add(self, (-1) * other)
+
+
+class Add(Energy):
+    """
+    Please not: If you add two operators which share some keys in the position
+    but have different values there, it is not guaranteed which value will be
+    used for the sum. You shouldn't do that anyways.
+    """
+    def __init__(self, op1, op2):
+        if isinstance(op1.position, MultiField) and isinstance(op2.position, MultiField):
+            a = op1.position._val
+            b = op2.position._val
+            # Note: In python >3.5 one could do {**a, **b}
+            ab = a.copy()
+            ab.update(b)
+            position = MultiField(ab)
+        elif isinstance(op1.position, Field) and isinstance(op2.position, Field):
+            position = op1.position
+        else:
+            raise TypeError
+        super(Add, self).__init__(position)
+
+        self._op1 = op1.at(position)
+        self._op2 = op2.at(position)
+
+    def at(self, position):
+        return self.__class__(self._op1.at(position), self._op2.at(position))
+
+    @property
+    @memo
+    def value(self):
+        return self._op1.value + self._op2.value
+
+    @property
+    @memo
+    def gradient(self):
+        return self._op1.gradient + self._op2.gradient
+
+    @property
+    @memo
+    def curvature(self):
+        return self._op1.curvature + self._op2.curvature