new_contents: from ..core import Expr
  from .core import Scalar
  from .numbers import NumberInterface
  from .boolean import BooleanInterface
  from datashape import dshape
  
  class ScalarSymbol(NumberInterface, BooleanInterface):
      __slots__ = 'name', 'dtype'
  
      def __init__(self, name, dtype='real'):
          self.name = name
          self.dtype = dtype
  
      @property
      def dshape(self):
          return dshape(self.dtype)
  
      def __str__(self):
          return str(self.name)
  
      __hash__ = Expr.__hash__
  
subject: add missing interface file