text: import sympy as sp
  
  
  class Material(object):
      def __init__(self, name, refractive_index_value=None):
          self.name = name
          self.n_symbol = sp.Symbol("n_"+name)
          self.n_value = refractive_index_value
  
      @property
      def substitutions(self):
          if self.n_value is None:
              return []
          else:
              return [(self.n_symbol, self.n_value)]
  