diff --git a/src/tequila/circuit/circuit.py b/src/tequila/circuit/circuit.py
index de47b77a..57bd3d99 100644
--- a/src/tequila/circuit/circuit.py
+++ b/src/tequila/circuit/circuit.py
@@ -9,6 +9,7 @@
 
 from .qpic import export_to
 
+
 class QCircuit():
     """
     Fundamental class representing an abstract circuit.
@@ -254,8 +255,8 @@ def replace_gates(self, positions: list, circuits: list, replace: list = None):
             the gates to add at the corresponding positions
         replace: list of bool: (Default value: None)
             Default is None which corresponds to all true
-            decide if gates shall be replaces or if the new parts shall be inserted without replacement
-            if replace[i] = true: gate at position [i] will be replaces by gates[i]
+            decide if gates shall be replaced or if the new parts shall be inserted without replacement
+            if replace[i] = true: gate at position [i] will be replaced by gates[i]
             if replace[i] = false: gates[i] circuit will be inserted at position [i] (beaming before gate previously at position [i])
         Returns
         -------
@@ -271,8 +272,9 @@ def replace_gates(self, positions: list, circuits: list, replace: list = None):
         dataset = zip(positions, circuits, replace)
         dataset = sorted(dataset, key=lambda x: x[0])
 
-        offset = 0
-        new_gatelist = self.gates
+        new_gatelist = []
+        last_idx = -1
+
         for idx, circuit, do_replace in dataset:
 
             # failsafe
@@ -283,13 +285,14 @@ def replace_gates(self, positions: list, circuits: list, replace: list = None):
             else:
                 gatelist = [circuit]
 
-            pos = idx + offset
-            if do_replace:
-                new_gatelist = new_gatelist[:pos] + gatelist + new_gatelist[pos + 1:]
-                offset += len(gatelist) - 1
-            else:
-                new_gatelist = new_gatelist[:pos] + gatelist + new_gatelist[pos:]
-                offset += len(gatelist)
+            new_gatelist += self.gates[last_idx + 1:idx]
+            new_gatelist += gatelist
+            if not do_replace:
+                new_gatelist.append(self.gates[idx])
+
+            last_idx = idx
+
+        new_gatelist += self.gates[last_idx + 1:]
 
         result = QCircuit(gates=new_gatelist)
         result.n_qubits = max(result.n_qubits, self.n_qubits)
@@ -386,7 +389,7 @@ def __iadd__(self, other):
         for k, v in other._parameter_map.items():
             self._parameter_map[k] += [(x[0] + offset, x[1]) for x in v]
 
-        self._gates += copy.deepcopy(other.gates)
+        self._gates += other.gates
         self._min_n_qubits = max(self._min_n_qubits, other._min_n_qubits)
 
         return self
@@ -593,7 +596,7 @@ def _inpl_control_circ(self, control) -> None:
 
         This is an in-place method, so it mutates self and doesn't return any value.
 
-        Raise TequilaWarning if there any qubits in common between self and control.
+        Raise TequilaWarning if there are any qubits in common between self and control.
         """
         gates = self.gates
         control = list_assignment(control)
@@ -610,8 +613,10 @@ def _inpl_control_circ(self, control) -> None:
                 if len(control_lst) < len(gate.control) + len(control):
                     # warnings.warn("Some of the controls {} were already included in the control "
                     #               "of a gate {}.".format(control, gate), TequilaWarning)
-                    raise TequilaWarning(f'Some of the controls {control} were already included '
-                                         f'in the control of a gate {gate}.')
+                    raise TequilaWarning(f"Some of the controls {control} were already included "
+                                         f"in the control of a gate {gate}. "
+                                         f"This might be because the same instance of a gate is used in multiple places, "
+                                         f"e.g. because the same circuit was appended twice.")
             else:
                 control_lst, not_control = list(control), list()
 
diff --git a/src/tequila/utils/bitstrings.py b/src/tequila/utils/bitstrings.py
index 9ae7cb20..3db1293d 100644
--- a/src/tequila/utils/bitstrings.py
+++ b/src/tequila/utils/bitstrings.py
@@ -69,6 +69,12 @@ def integer(self, other: int):
         self.update_nbits()
         return self
 
+    def to_integer(self, numbering: BitNumbering):
+        if numbering == self.numbering:
+            return self.integer
+        else:
+            return reverse_int_bits(self.integer, self.nbits)
+
     @property
     def array(self):
         return [int(i) for i in self.binary]
diff --git a/src/tequila/wavefunction/qubit_wavefunction.py b/src/tequila/wavefunction/qubit_wavefunction.py
index e7f48ad9..1c700d1c 100644
--- a/src/tequila/wavefunction/qubit_wavefunction.py
+++ b/src/tequila/wavefunction/qubit_wavefunction.py
@@ -365,7 +365,7 @@ def normalize(self, inplace: bool = False) -> QubitWaveFunction:
     # because the __mul__ implementation of the number tries to perform some sort of array
     # operation.
     def length(self):
-        return sum(1 for _ in self.raw_items())
+        return sum(1 for (k, v) in self.raw_items() if abs(v) > 1e-6)
 
     def __repr__(self):
         result = str()