Symbol extends Annotatable A single symbol. This is the atomic unit of a SymbolList, or a sequence.
Some symbols represent a single token in the sequence.
For example, there is a Symbol instance for adenine in DNA, and another one for cytosine. Symbols can potentialy represent sets of Symbols. For example, n represents any DNA Symbol, and X any protein Symbol. Gap represents the knowledge that there is no Symbol. In addition, some symbols represent ordered lists of other Symbols.