code: def visit_Placeholder(self, pattern):
          
          if (pattern.id in self.placeholders and
                  not Check(self.node, self.placeholders).visit(
                      self.placeholders[pattern.id])):
              return False
          else:
              self.placeholders[pattern.id] = self.node
              return True
docs: Save matching node or compare it with the existing one.
  
          FIXME : What if the new placeholder is a better choice?