public LinearEquationException(string message, Exception innerException, Equation source)
     : base(message, innerException)
 {
 }
 public EquationException(string message, Equation source)
     : base(message)
 {
     _source = source;
 }
 public LinearEquationException(string message, Equation source)
     : base(message, source)
 {
     HelpLink = "https://en.wikipedia.org/wiki/Linear_equation";
 }