code: def descriptor_for_symbol(self, symbol):
          
          if isinstance(symbol, basestring):
              symbol = sympy.Symbol(symbol)
  
          try:
              return self._descriptions_dict[symbol]
          except KeyError:
              raise KeyError("Symbol {0!r} not found in left-hand-side of the equations".format(symbol))
docs: Given the symbol associated with the problem.
          Returns the :class:`~means.core.descriptors.Descriptor` associated with that symbol
  
          :param symbol: Symbol
          :type symbol: basestring|:class:`sympy.Symbol`
          :return: