A constraint is a function that computes a degree of violation that is managed as any invariant.
A constraint system is a composition of constraints.
constraints left == right this is considered as a primitive constraint and used in the Constraint class, so that it is part of the core instead of the library
This package defines a core structure for constraints. A Constraint is to be posted in ConstraintSystem All this mechanics aims at computing violation degrees associated with constraints and with constraint systems.