The Key concept behind this new MC-SYM is the Fragment Generator (FG).
A Fragment Generator is an abstract object responsible of the structure of a
specific fragment (a set of residues). When one asks for it, a FG will try
to generate a new structure for its subordinate fragment. Many kinds of FGs
are implemented, each of them generating structures in its own way. The act
of asking a FG for a new structure is said to be an Advance call on
this FG (We also say ``to call Advance'' on a FG). Some Fragment Generators
can even use other FGs to construct sub-fragment structures.
Typically, one can define algoritms on FGs without knowing how they generate
sub-structures. For example, it is possible to implement a Backtracking algorithm on FGs, independently of the types of those
FGs. Actually, this is implemented using a Backtrack Fragment Generator.
Such a BacktrackFG will backtrack on the states of his subordinate FGs, i.e.
it will test every combination of the states of its subordinates FGs.
Actually, all modeling projects require at least one Backtrack Fragment
Generator.
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