public LorentzVectorVariableU(Variable variable0, Variable variable1, Variable variable2, Variable variable3)
     : base(variable0, variable1, variable2, variable3)
 {
     this.variables[0] = variable0;
     this.variables[1] = variable1;
     this.variables[2] = variable2;
     this.variables[2] = variable3;
     this.Del          = new LorentzVectorOperatorL(new Derivative(variable0), new Derivative(variable1), new Derivative(variable2), new Derivative(variable3));
 }
 public EuclideanMatrix4 Curl(LorentzVectorOperatorL del)
 {
     return(new EuclideanMatrix4(MatrixUtilities.MatrixCrossProduct(i => del[i], i => this[i], (x, y) => x - y, (x, y) => x * y)));
 }