/* --- *
V a r i a b l e
--- 
A constrained variable. In addition to its value, it maintain the
structure of the constraint graph, the current dataflow graph, and
various parameters of interest to the DeltaBlue incremental
constraint solver.
function Variable(name, initialValue) {
  this.value = initialValue || 0;
  this.constraints = new OrderedCollection();
  this.determinedBy = null;
  this.mark = 0;
  this.walkStrength = Strength.WEAKEST;
  this.stay = true;
  this.name = name;
}