new_contents: import pickle
  from indra.assemblers import PysbAssembler
  
  class IncrementalModel(object):
      def __init__(self, fname=None):
          if fname is None:
              self.stmts = {}
          else:
              try:
                  self.stmts = pickle.load(open(fname, 'rb'))
              except:
                  print 'Could not load %s' % fname
                  self.stmts = {}
  
      def save(self, fname='model.pkl'):
          with open(fname, 'wb') as fh:
              pickle.dump(self.stmts, fh)
  
      def add_statements(self, pmid, stmts):
          self.stmts[pmid] = stmts
  
      def get_statements(self):
          stmt_lists = [v for k, v in self.stmts.iteritems()]
          stmts = []
          for s in stmt_lists:
              stmts += s
          return stmts
  
      def make_model(self):
          pa = PysbAssembler()
          pa.add_statements(self.get_statements())
          pa.make_model()
          return pa.model
  
subject: Add IncrementalModel