code: def __eliminate_unused_constraits (self, objects):
          
          result = []
          for c in self.constraints_:
              if c [0] in objects and c [1] in objects:
                  result.append (c)
  
          return result
docs: Eliminate constraints which mention objects not in 'objects'.
              In graph-theory terms, this is finding subgraph induced by
              ordered vertices.