Extend not defined in lib/serialize/json/serializeMapping.js

[marcoexplora]

I noted that in lib/serialize/json/serializeMapping.js extend function is used but not defined and create this error:

ReferenceError: extend is not defined at serializeMapping (serializeMapping.js?2c2f:46) at eval (serializeModule.js?1154:45) at Array.map (<anonymous>) at serializeModule (serializeModule.js?1154:44) at eval (serializeModuleDefinition.js?63b0:51) at Array.map (<anonymous>) at serializeModuleDefinition (serializeModuleDefinition.js?63b0:50) at eval (serializeJSON.js?40b2:53) at Array.map (<anonymous>) at SBOLDocument.serializeJSON (serializeJSON.js?40b2:52)