code: def children_are_up_to_date(self):
          
          # Allow the children to calculate their signatures.
          self.binfo = self.get_binfo()
          if self.always_build:
              return None
          state = 0
          for kid in self.children(None):
              s = kid.get_state()
              if s and (not state or s > state):
                  state = s
          return (state == 0 or state == SCons.Node.up_to_date)
docs: Alternate check for whether the Node is current:  If all of
          our children were up-to-date, then this Node was up-to-date, too.
  
          The SCons.Node.Alias and SCons.Node.Python.Value subclasses
          rebind their current() method to this method.