docstr: Return a tuple of free variables in the function.
          
code: def get_frees(self):
      """Return a tuple of free variables in the function."""
      if self.__frees is None:
          is_free = lambda x: ((x >> SCOPE_OFF) & SCOPE_MASK) == FREE
          self.__frees = self.__idents_matching(is_free)
      return self.__frees
  
imports: import _symtable
  from _symtable import USE, DEF_GLOBAL, DEF_NONLOCAL, DEF_LOCAL, DEF_PARAM, DEF_IMPORT, DEF_BOUND, DEF_ANNOT, SCOPE_OFF, SCOPE_MASK, FREE, LOCAL, GLOBAL_IMPLICIT, GLOBAL_EXPLICIT, CELL
  import weakref
label_desc: Use image node_id 6 for calling the Function obj's underlying member method code with example usage: obj.get_frees() and returns: self