ass invariants and old values in postconditions, not to mention the lack
      of more advanced features like subcontracting. All these features are instead
      supported by this library (see <a class="link" href="boost_contract/contract_programming_overview.html#boost_contract.contract_programming_overview.feature_summary" title="Feature Summary">Feature
      Summary</a> for a detailed comparison between the features supported by
      this library and the ones listed in different contract programming p