/// <summary>
/// Evaluate this parse tree node with the given variable values.
/// </summary>
/// <param name="variables">The variable values to use in calculation.</param>
/// <returns>Returns the result of the evaluation.</returns>
public override double Evaluate(VariableSet variables)
{
double operandResult = Operand.Evaluate(variables);
return(Evaluator.Function(operandResult));
}