diff --git a/Common/engine_interface_base.h b/Common/engine_interface_base.h
index 547b2df..e71ed99 100644
--- a/Common/engine_interface_base.h
+++ b/Common/engine_interface_base.h
@@ -40,21 +40,21 @@ public:
 	InterpolationType GetInterpolationType() {return m_InterpolType;}
 
 	//! Get the (interpolated) electric field at \p pos. \sa SetInterpolationType
-	virtual double* GetEField(const unsigned int* pos, double* out) const {UNUSED(pos); return out;}
+	virtual double* GetEField(const unsigned int* pos, double* out) const =0;
 	//! Get the (interpolated) magnetic field at \p pos. \sa SetInterpolationType
-	virtual double* GetHField(const unsigned int* pos, double* out) const {UNUSED(pos); return out;}
+	virtual double* GetHField(const unsigned int* pos, double* out) const =0;
 
 	//! Calculate the electric field integral along a given line
-	virtual double CalcVoltageIntegral(const unsigned int* start, const unsigned int* stop) const {UNUSED(start); UNUSED(stop); return 0.0;}
+	virtual double CalcVoltageIntegral(const unsigned int* start, const unsigned int* stop) const =0;
 
 	//! Convert the interpolation type into a string.
 	static std::string GetInterpolationNameByType(InterpolationType mode);
 
 	//! Get the current simulation time
-	virtual double GetTime(bool dualTime=false) const {UNUSED(dualTime); return 0.0;}
+	virtual double GetTime(bool dualTime=false) const =0;
 
 	//! Get the current number of timesteps
-	virtual unsigned int GetNumberOfTimesteps() const {return 0;}
+	virtual unsigned int GetNumberOfTimesteps() const =0;
 
 protected:
 	Engine_Interface_Base();
diff --git a/Common/operator_base.h b/Common/operator_base.h
index a2842bc..523cb14 100644
--- a/Common/operator_base.h
+++ b/Common/operator_base.h
@@ -22,51 +22,52 @@
 #include "Common/processing.h"
 #include "string"
 
+//! Abstract base-class for a common operator
 class Operator_Base
 {
 public:
 	//! Get the timestep used by this operator
-	double GetTimestep() const {return dT;};
+	virtual double GetTimestep() const {return dT;};
 
 	//! Get the number of cells or nodes defined by this operator
-	virtual double GetNumberCells() const {return 0;}
+	virtual double GetNumberCells() const =0;
 
 	//! Get the number of timesteps satisfying the nyquist condition (may depend on the excitation)
-	unsigned int GetNumberOfNyquistTimesteps() const {return 0;}
+	virtual unsigned int GetNumberOfNyquistTimesteps() const =0;
 
 	//! Returns the number of lines as needed for post-processing etc. (for the engine, use GetOriginalNumLines())
-	virtual unsigned int GetNumberOfLines(int ny) const {return numLines[ny];}
+	virtual unsigned int GetNumberOfLines(int ny) const =0;
 
 	//! Get the name for the given direction: 0 -> x, 1 -> y, 2 -> z
 	virtual std::string GetDirName(int ny) const;
 
 	//! Get the grid drawing unit in m
-	virtual double GetGridDelta() const {return 0;}
+	virtual double GetGridDelta() const =0;
 
 	//! Get the mesh delta times the grid delta for a 3D position (unit is meter)
-	virtual double GetMeshDelta(int n, const unsigned int* pos, bool dualMesh=false) const {UNUSED(n); UNUSED(pos); UNUSED(dualMesh); return 0.0;}
+	virtual double GetMeshDelta(int n, const unsigned int* pos, bool dualMesh=false) const =0;
 
 	//! Get the disc line in \a n direction (in drawing units)
-	virtual double GetDiscLine(int n, unsigned int pos, bool dualMesh=false) const {UNUSED(n); UNUSED(pos); UNUSED(dualMesh); return 0.0;}
+	virtual double GetDiscLine(int n, unsigned int pos, bool dualMesh=false) const =0;
 
 	//! Get the node width for a given direction \a n and a given mesh position \a pos
-	virtual double GetNodeWidth(int ny, const unsigned int pos[3], bool dualMesh = false) const {UNUSED(ny); UNUSED(pos); UNUSED(dualMesh); return 0.0;}
+	virtual double GetNodeWidth(int ny, const unsigned int pos[3], bool dualMesh = false) const =0;
 
 	//! Get the node area for a given direction \a n and a given mesh position \a pos
-	virtual double GetNodeArea(int ny, const unsigned int pos[3], bool dualMesh = false) const {UNUSED(ny); UNUSED(pos); UNUSED(dualMesh); return 0.0;}
+	virtual double GetNodeArea(int ny, const unsigned int pos[3], bool dualMesh = false) const =0;
 
 	//! Get the length of an FDTD edge (unit is meter).
-	virtual double GetEdgeLength(int ny, const unsigned int pos[3], bool dualMesh = false) const {UNUSED(ny); UNUSED(pos); UNUSED(dualMesh); return 0.0;}
+	virtual double GetEdgeLength(int ny, const unsigned int pos[3], bool dualMesh = false) const =0;
 
 	//! Get the area around an edge for a given direction \a n and a given mesh posisition \a pos
 	/*!
 		This will return the area around an edge with a given direction, measured at the middle of the edge.
 		In a cartesian mesh this is equal to the NodeArea, may be different in other coordinate systems.
 	*/
-	virtual double GetEdgeArea(int ny, const unsigned int pos[3], bool dualMesh = false) const {UNUSED(ny); UNUSED(pos); UNUSED(dualMesh); return 0.0;}
+	virtual double GetEdgeArea(int ny, const unsigned int pos[3], bool dualMesh = false) const =0;
 
 	//! Snap the given coodinates to mesh indices
-	virtual bool SnapToMesh(double* coord, unsigned int* uicoord, bool lower=false, bool* inside=NULL) {UNUSED(coord); UNUSED(uicoord); UNUSED(lower); UNUSED(inside); return false;};
+	virtual bool SnapToMesh(const double* coord, unsigned int* uicoord, bool lower=false, bool* inside=NULL) const =0;
 
 	//! Set the boundary conditions
 	virtual void SetBoundaryCondition(int* BCs) {for (int n=0; n<6; ++n) m_BC[n]=BCs[n];}
diff --git a/Common/readme.txt b/Common/readme.txt
index 37e9a22..8c62be8 100644
--- a/Common/readme.txt
+++ b/Common/readme.txt
@@ -1,6 +1,6 @@
-readme for openEMS/Commen
+readme for openEMS/Common
 
-- This folder contains all classes common for all numerical solver included in openEMS (currently only FDTD)
+- This folder contains all classes common for all numerical solver included in openEMS (currently only EC-FDTD)
 	- Operator-Base class
 	- Engine-Interface classes
-	- Commen processing classes
+	- Common processing classes
diff --git a/FDTD/operator.cpp b/FDTD/operator.cpp
index 11c7f8f..ab666f9 100644
--- a/FDTD/operator.cpp
+++ b/FDTD/operator.cpp
@@ -124,7 +124,7 @@ double Operator::GetNodeArea(int ny, const unsigned int pos[3], bool dualMesh) c
 	return GetNodeWidth(nyP,pos,dualMesh) * GetNodeWidth(nyPP,pos,dualMesh);
 }
 
-bool Operator::SnapToMesh(double* dcoord, unsigned int* uicoord, bool lower, bool* inside)
+bool Operator::SnapToMesh(const double* dcoord, unsigned int* uicoord, bool lower, bool* inside) const
 {
 	bool ok=true;
 	unsigned int numLines[3];
diff --git a/FDTD/operator.h b/FDTD/operator.h
index 24103df..06edb41 100644
--- a/FDTD/operator.h
+++ b/FDTD/operator.h
@@ -28,7 +28,7 @@ class Operator_Extension;
 class Engine;
 class TiXmlElement;
 
-//! Abstract base-class for the FDTD-operator
+//! Basic FDTD-operator
 class Operator : public Operator_Base
 {
 	friend class Engine;
@@ -82,7 +82,9 @@ public:
 	bool GetTimestepValid() const {return !m_InvaildTimestep;}
 	virtual double GetNumberCells() const;
 
-	unsigned int GetNumberOfNyquistTimesteps() const {return Exc->GetNyquistNum();}
+	virtual unsigned int GetNumberOfNyquistTimesteps() const {return Exc->GetNyquistNum();}
+
+	virtual unsigned int GetNumberOfLines(int ny) const {return numLines[ny];}
 
 	//! Returns the number of lines as needed for the engine etc. (for post-processing etc, use GetNumLines())
 	virtual unsigned int GetOriginalNumLines(int ny) const {return numLines[ny];}
@@ -113,7 +115,7 @@ public:
 	*/
 	virtual double GetEdgeArea(int ny, const unsigned int pos[3], bool dualMesh = false) const {return GetNodeArea(ny,pos,dualMesh);}
 
-	virtual bool SnapToMesh(double* coord, unsigned int* uicoord, bool lower=false, bool* inside=NULL);
+	virtual bool SnapToMesh(const double* coord, unsigned int* uicoord, bool lower=false, bool* inside=NULL) const;
 
 	virtual void AddExtension(Operator_Extension* op_ext);
 	virtual size_t GetNumberOfExtentions() const {return m_Op_exts.size();}