Change internal representation of Heading.
Split Constraint to a Hierarchy.
Indentation fix.
Fix error message.
Use default Mocha reporter.
Use 'cause = last error' strategy for nicer error messages
Attempt to improve error messages when objects are toStringed.
Fix BuiltinType#include to support sub types/prototypes
Add information contracts to first-class citizen
Refactor System's internal representation.
Add an information contract to System.
Fix a step about hierarchy.
Install a better delegation pattern between Type and its subclasses.
Fix a few hierarchy (test) problems.
Use ProxyTypes per default when compiling type references.