class_docstr: A class for representing multivariate rational function fields. 
docstr: Returns factorial of ``a``. 
code: def factorial(self, a):
      """Returns factorial of ``a``."""
      return self.dtype(self.domain.factorial(a))
  
imports: from sympy.polys.domains.compositedomain import CompositeDomain
  from sympy.polys.domains.field import Field
  from sympy.polys.polyerrors import CoercionFailed, GeneratorsError
  from sympy.utilities import public
label_desc: Use image node_id 31 for calling the FractionField obj's underlying member method code with example usage: obj.factorial(a) and returns: self