diff --git a/ODElibrary/AbstractSolver.cpp b/ODElibrary/AbstractSolver.cpp
index 3da9817..a77df61 100644
--- a/ODElibrary/AbstractSolver.cpp
+++ b/ODElibrary/AbstractSolver.cpp
@@ -1,78 +1,82 @@
 //
 // Created by lionel on 25.11.19.
 //
 
 #include <cassert>
 #include <iostream>
 #include "AbstractSolver.h"
 
 
 
 AbstractSolver::AbstractSolver(){
     stepSize=0;
     initialTime=0;
     finalTime=0;
     dimension=0;
     nSteps=0;
 };
 
 AbstractSolver::~AbstractSolver(){};
 
 void AbstractSolver::setInitialTime(const double t0) {
     initialTime = t0;
 }
 
 void AbstractSolver::setFinalTime(const double tf) {
     finalTime = tf;
 }
 
-void AbstractSolver::setStepSize(double h) {
-    stepSize = h;
+void AbstractSolver::setStepSize(const double h) {
+    if (h > 0) {
+        stepSize = h;
+    } else {
+        throw std::domain_error("Step size h must be strictly positive");
+    }
 }
 
 void AbstractSolver::setNumberOfSteps(int n){
     nSteps=n;
 }
 
 void AbstractSolver::setSpaceDimension(int dim) {
     assert(dim > 0);
     dimension=dim;
 }
 void AbstractSolver::setInitialValue(Eigen::VectorXd & y0) {
     initialValue = y0;
 }
 
 void AbstractSolver::setRightHandSide(Eigen::MatrixXd & rhsMatrix, Eigen::VectorXd (*g_)(double t)) {
     A = rhsMatrix;
     g = g_;
 }
 
 double AbstractSolver::getInitialTime() const{
     return initialTime;
 }
 
 double AbstractSolver::getFinalTime() const{
     return finalTime;
 }
 
 double AbstractSolver::getStepSize() const{
     return stepSize;
 }
 
 int AbstractSolver::getSpaceDimension() const {
     return dimension;
 }
 
 
 void AbstractSolver::setOutputType(outputTypes type) {
     outputType = type;
 }
 
 void AbstractSolver::sendToOutput(double t, Eigen::VectorXd y) {
     switch(outputType){
         case outputTypes::Terminal:
             std::cout << t << "\t" << y.transpose() << std::endl;
     }
 }
 
 
diff --git a/ODElibrary/AbstractSolver.h b/ODElibrary/AbstractSolver.h
index 4a16192..0dd57b5 100644
--- a/ODElibrary/AbstractSolver.h
+++ b/ODElibrary/AbstractSolver.h
@@ -1,61 +1,61 @@
 //
 // Created by lionel on 25.11.19.
 //
 
 #ifndef ODELIBRARY_ABSTRACTSOLVER_H
 #define ODELIBRARY_ABSTRACTSOLVER_H
 
 
 #include <vector>
 #include <Eigen/Dense>
 
 
 
 enum class outputTypes{ Unknown, File, Terminal};
 
 /** This is an abstract class for the solver of differential equations
  *
  */
 class AbstractSolver {
 protected:
     // variables for the setting
     double stepSize;
     double initialTime;
     double finalTime;
     int dimension; // dimension of x
     int nSteps;
     Eigen::VectorXd initialValue;
     outputTypes outputType;
 
     // variables for the rhs of the equation
     Eigen::MatrixXd A;
     Eigen::VectorXd (*g)(double t); // function g depending on time
 
 public:
     AbstractSolver();
     virtual ~AbstractSolver();
 
     void setInitialTime(double t0);
     void setFinalTime(double tf);
     void setInitialValue(Eigen::VectorXd & y0);
     void setStepSize(double h);
     void setNumberOfSteps(int n);
     void setSpaceDimension(int dim);
     void setRightHandSide(Eigen::MatrixXd & rhsMatrix, Eigen::VectorXd (*g_)(double t));
 
 
     void setOutputType(outputTypes type);
     void sendToOutput(double t, Eigen::VectorXd y);
 
     double getInitialTime() const;
     double getFinalTime() const;
     double getStepSize() const;
     int getSpaceDimension() const;
 
 
     virtual void solve() = 0;
 
 };
 
 
-#endif //ODELIBRARY_ABSTRACTSOLVER_H
+#endif ODELIBRARY_ABSTRACTSOLVER_H
diff --git a/ODElibrary/ForwardEuler.h b/ODElibrary/ForwardEuler.h
index f7349b3..b18f8e2 100644
--- a/ODElibrary/ForwardEuler.h
+++ b/ODElibrary/ForwardEuler.h
@@ -1,11 +1,14 @@
 
-
+#ifndef ODELIBRARY_FORWARDEULER_H
+#define ODELIBRARY_FORWARDEULER_H
 #include "AbstractSolver.h"
 
 
 class ForwardEuler : public AbstractSolver{
 public:
     void solve() override;
     void solve(std::ofstream& outputFile);
 };
 
+#endif ODELIBRARY_FORWARDEULER_H
+