code: def main_target_default_build (self, specification, project):
          
          assert is_iterable_typed(specification, basestring)
          assert isinstance(project, ProjectTarget)
          if specification:
              return property_set.create_with_validation(specification)
          else:
              return project.get ('default-build')
docs: Return the default build value to use when declaring a main target,
              which is obtained by using specified value if not empty and parent's
              default build attribute otherwise.
              specification:  Default build explicitly specified for a main target
              project:        Project where the main target is to be declared