code: def pretty_use_unicode(flag=None):
      """Set whether pretty-printer should use unicode by default"""
      global _use_unicode
      global unicode_warnings
      if flag is None:
          return _use_unicode
  
      if flag and unicode_warnings:
          # print warnings (if any) on first unicode usage
          warnings.warn(unicode_warnings)
          unicode_warnings = ""
  
      use_unicode_prev = _use_unicode
      _use_unicode = flag
      return use_unicode_prev
  
imports: import sys
  import warnings
  from string import ascii_lowercase, ascii_uppercase
  import unicodedata
  from sympy.printing.conventions import split_super_sub
  from sympy.core.alphabets import greeks
  from sympy.utilities.exceptions import sympy_deprecation_warning
label_desc: Use image node_id 2 for calling a global function with example usage: pretty_use_unicode(flag) and returns: use_unicode_prev, _use_unicode