Spec Dependent StringSet

StringSet implementation that only stores pointers to strings in other string sets.

/*!
 * @class DependentStringSet Dependent StringSet
 * 
 * @extends StringSet
 * 
 * @headerfile <seqan/sequence.h>
 * 
 * @brief StringSet implementation that only stores pointers to strings in other
 *        string sets.
 * 
 * @signature template <typename TString, typename TSpec> class
 *            StringSet<TString, Depedent<TSpec> >;
 * 
 * @tparam TString The type of the string to store in the string set.
 * @tparam TSpec Tag for further specializing the string set.
 * 
 * Important: This is an abstract class. Use one of the following
 * specializations: @link TightDependentStringSet @endlink and @link
 * GenerousDependentStringSet @endlink.
 * 
 * A Dependent StringSet (DSS) can be used like a normal StringSet while
 * internally storing only pointers to a source set.
 * 
 * @section Modifying a Dependent StringSet
 * 
 * (1) Removing a sequence from a DSS only removes the pointer and does not
 * change the source set.
 * 
 * (2) Appending a sequence to a DSS appends a pointer to that sequence and does
 * not change the source set.
 * 
 * (3) Assigning a sequence to a position (or id) of a DSS, dereferences the
 * pointer first and does thus additionally change the source set.
 * 
 * (4) Accessing the DSS at a position (or id) dereferences the pointer, and
 * when stored as a reference, modifications also lead to a change in the source
 * set.
 * 
 * @section Position vs. Id
 * 
 * When a sequence is removed in a DSS, the positions of pointers shift and do
 * not represent the exact position in the original source set anymore. To
 * distinguish between new and original positions, we introduce the term
 * <tt>id</tt> which refers to the original positions in the source set. Every
 * modification of a DSS can be either based on the id (function ending on
 * <tt>ById()</tt>) or the position depending on the behaviour you want to
 * realize.
 * 
 * The following figure illustrates the behaviour when removing a sequence:
 * 
 * <img src="position_vs_id.png" title="Impact to positions and ids on removal
 * of an entry in an DSS" width="900">
 * 
 * @section Tight vs. Generous
 * 
 * The two different specializations <tt>Tight</tt> and <tt>Generous</tt>
 * provide the same functionality but behave slightly different concerning run
 * time and certain errors (e.g. index out of range). See the correspoinding
 * documentation pages @link TightDependentStringSet @endlink and @link
 * GenerousDependentStringSet @endlink for further details.
 */