code: function checkTypeComparableTo(source, target, errorNode, headMessage, containingMessageChain) {
              return checkTypeRelatedTo(source, target, comparableRelation, errorNode, headMessage, containingMessageChain);
          }
docs: This is *not* a bi-directional relationship.
  If one needs to check both directions for comparability, use a second call to this function or 'isTypeComparableTo'.