class_docstr: A class for representing multivariate polynomial rings. 
docstr: Returns ``True`` if ``a`` is a unit of ``self``
docstr_tok: ["Returns","``","True","``","if","``","a","``","is","a","unit","of","``","self","``"]
code: def is_unit(self, a):
      """Returns ``True`` if ``a`` is a unit of ``self``"""
      if not a.is_ground:
          return False
      K = self.domain
      return K.is_unit(K.convert_from(a, self))
  
imports: from sympy.polys.domains.ring import Ring
  from sympy.polys.domains.compositedomain import CompositeDomain
  from sympy.polys.polyerrors import CoercionFailed, GeneratorsError
  from sympy.utilities import public
label_desc: Use image node_id 9 for calling the PolynomialRing obj's underlying member method code with example usage: obj.is_unit(a) and returns: K, False