code: def add_source(self, source):
          
          if self._specific_sources:
              return
          try:
              self._add_child(self.sources, self.sources_set, source)
          except TypeError as e:
              e = e.args[0]
              if SCons.Util.is_List(e):
                  s = list(map(str, e))
              else:
                  s = str(e)
              raise SCons.Errors.UserError("attempted to add a non-Node as source of %s:\n\t%s is a %s, not a Node" % (str(self), s, type(e)))
docs: Adds sources.