code: def _subsets(n):
      """
      Returns all possible subsets of the set (0, 1, ..., n-1) except the
      empty set, listed in reversed lexicographical order according to binary
      representation, so that the case of the fourth root is treated last.
  
      Examples
      ========
  
      >>> from sympy.simplify.sqrtdenest import _subsets
      >>> _subsets(2)
      [[1, 0], [0, 1], [1, 1]]
  
      """
      if n == 1:
          a = [[1]]
      elif n == 2:
          a = [[1, 0], [0, 1], [1, 1]]
      elif n == 3:
          a = [
              [1, 0, 0],
              [0, 1, 0],
              [1, 1, 0],
              [0, 0, 1],
              [1, 0, 1],
              [0, 1, 1],
              [1, 1, 1],
          ]
      else:
          b = _subsets(n - 1)
          a0 = [x + [0] for x in b]
          a1 = [x + [1] for x in b]
          a = a0 + [[0] * (n - 1) + [1]] + a1
      return a
  
imports: from sympy.core import Add, Expr, Mul, S, sympify
  from sympy.core.function import _mexpand, count_ops, expand_mul
  from sympy.core.sorting import default_sort_key
  from sympy.core.symbol import Dummy
  from sympy.functions import root, sign, sqrt
  from sympy.polys import Poly, PolynomialError
label_desc: Use image node_id 4 for calling a global function with example usage: _subsets(n) and returns: a