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By applying the motto from Existentialism that “existence precedes essence”, a common approach is possible to topics as diverse as mixins, data integration using semantic mapping, code integration of uncooperative APIs and Frameworks, dynamic object "roles", open classes and external methods, ontology mediation, and object evolution & class/API versioning.
In a nutshell, Existential Programming proposes that one develop data models, object models, programming frameworks, etc. without imposing a single ontology, as would normally be the case; in other words, without imposing a single master E/R data model, O/O class hierarchy, business domain model, etc.
The Zen-like thought here is: Until one can create a type-less object, one can not create an object which can be multiple types simultaneously.