code: def set_pkg_supplier(self, doc, entity):
          
          self.assert_package_exists()
          if not self.package_supplier_set:
              self.package_supplier_set = True
              if validations.validate_pkg_supplier(entity):
                  doc.package.supplier = entity
                  return True
              else:
                  raise SPDXValueError('Package::Supplier')
          else:
              raise CardinalityError('Package::Supplier')
docs: Sets the package supplier, if not already set.
          entity - Organization, Person or NoAssert.
          Raises CardinalityError if already has a supplier.
          Raises OrderError if no package previously defined.