code: def subs(a, b):
      """Replace expressions exactly"""
  
      def subs_rl(expr):
          if expr == a:
              return b
          else:
              return expr
  
      return subs_rl
  
file_path: sympy/sympy/strategies/rl.py
imports: from sympy.utilities.iterables import sift
  from .util import new
label_desc: Use image node_id 5 for calling a global function with example usage: subs(a, b) and returns: subs_rl, b, expr