code: def sources (self):
          
          if self.source_targets_ == None:
              self.source_targets_ = []
              for s in self.sources_:
                  self.source_targets_.append(resolve_reference(s, self.project_)[0])
  
          return self.source_targets_
docs: Returns the list of AbstractTargets which are used as sources.
              The extra properties specified for sources are not represented.
              The only used of this rule at the moment is the '--dump-tests'
              feature of the test system.