/// <summary>
/// Returns a <see cref="String"/> that represents the <see cref="Variable"/> with the
/// specified identifier in the specified collection.</summary>
/// <param name="variables">
/// The <see cref="VariableList"/> to search.</param>
/// <param name="id">
/// The <see cref="Variable.Id"/> string of the <see cref="Variable"/> instance to
/// format.</param>
/// <returns><para>
/// The result of <see cref="Variable.ToString"/> for the <paramref name="variables"/>
/// element with the specified <paramref name="id"/>.
/// </para><para>-or-</para><para>
/// A null reference if no such element is found.</para></returns>
private static string FormatVariable(VariableList variables, string id)
{
int index = variables.IndexOfKey(id);
return(index < 0 ? null : variables[index].ToString());
}