docs: def expr(self, items):
          """Return the sum of each name multiplied by a coefficient.
  
          >>> v = prob.namespace(name='v')
          >>> v.define(['a', 'b', 'c'], lower=0, upper=10)
          >>> prob.set_objective(v.expr([('a', 2), ('b', 1)]))
          """
          return Expression({(self, name): value for name, value in items})