code: def build_sub_expression(name, ne_ref=None, operator='union'):
          
          ne_ref = [] if ne_ref is None else ne_ref
          json = {'name': name,
                  'ne_ref': ne_ref,
                  'operator': operator}
          return json
docs: Static method to build and return the proper json for a sub-expression.
          A sub-expression would be the grouping of network elements used as a
          target match. For example, (network A or network B) would be considered
          a sub-expression. This can be used to compound sub-expressions before
          calling create.
  
          :param str name: name of sub-expression
          :param list ne_ref: network elements references
          :param str operator: exclusion (negation), union, intersection (default: union)
          :return: JSON of subexpression. Use in :func:`~create` constructor