class_docstr: A class for representing multivariate polynomial rings. 
code: def canonical_unit(self, a):
      u = self.domain.canonical_unit(a.LC)
      return self.ring.ground_new(u)
  
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 10 for calling the PolynomialRing obj's underlying member method code with example usage: obj.canonical_unit(a) and returns: self