ConstraintError is used to return detailed violation information from CheckConstraints.

Dependency is used to create a directed edge between two nouns.

Graph is used to represent a dependency graph.

Nouns are the key structure of the dependency graph.

ValidateError implements the Error interface but provides additional information on a validation error.

Violation is used to pass along information about a constraint violation.

Constraint is used by dependencies to allow arbitrary constraints between nouns.

WalkFunc is the type used for the callback for Walk.