ConstraintExpression represents a compound constraint in the process of being constructed from a series of syntactic elements. Individual elements are appended to the expression as they are reorganized. When a constraint is appended, it is returned as the value of the operation so that modifiers may be applied. However, any partially built expression is attached to the constraint for later resolution. When an operator is appended, the partial expression is returned. If it's a self-resolving operator, then a ResolvableConstraintExpression is returned.