diff --git a/parallel.py b/parallel.py
index 5da4329..5a50dbf 100644
--- a/parallel.py
+++ b/parallel.py
@@ -501,7 +501,7 @@ class PPP:
             self.init_mirror_symmetry(key,axis,+1)
         return
 
-    def init_mirror_symmetry(self,key,axis,wall):
+    def init_mirror_symmetry(self,key,axis,wall,invert=False):
         '''Applies symmetry: [u,v,w](x,-y,z) -> [u,-v,w](x,y,z)
            Proposed by Lozano-Duran JFM 2016 and used for free-slip.'''
         if self.periodicity[axis]: 
@@ -515,33 +515,14 @@ class PPP:
         sl[axis] = iwall
         sl = tuple(sl)
         if key==keyu[axis]:
-            self.symmetries[key][sl] = -1
+            self.symmetries[key][sl] = -1 if not invert else +1
         else:
-            self.symmetries[key][sl] = 1
+            self.symmetries[key][sl] = +1 if not invert else -1
         return
 
-    def set_noslip_bc(self,axis,wall):
-        '''Applies symmetry: [u,v,w](x,-y,z) -> [-u,v,-w](x,y,z)
-           The boundary conditions are only applied to fields which have already been loaded!'''
-        if self.periodicity[axis]:
-            return
-        if wall in (-1,'low'): iwall = 0
-        elif wall in (1,'high'): iwall = 2
-        else: raise ValueError('Invalid value for "wall". Valid values are -1 / "low" or 1 / "high".')
-        keyu = ('u','v','w')
-        for key in keyu:
-            if not key in self.symmetries:
-                continue
-            # Get position in symmetries array to be edited
-            sl = [1,1,1]
-            sl[axis] = iwall
-            sl = tuple(sl)
-            if key==keyu[axis]:
-                self.symmetries[key][sl] = 1
-            else:
-                self.symmetries[key][sl] = -1
-            self.impose_boundary_conditions(key)
-        return
+    def init_inversion_symmetry(self,key,axis,wall):
+        '''Applies symmetry: [u,v,w](x,-y,z) -> [-u,v,-w](x,y,z)'''
+        return self.init_mirror_symmetry(key,axis,wall,invert=True)
 
     def copy(self,key,key_out,skip_data=False,skip_symmetries=False):
         '''Copies a field to a new key.'''