input: Write a Python function to parse and evaluate a simple mathematical expression in postfix notation (Reverse Polish Notation).
  assert expression_parser("3 4 +") == 7
  assert expression_parser("10 2 8 * + 3 -") == 23
  assert expression_parser("2 3 4 * +") == 14
output: def expression_parser(expression):
      stack = []
      tokens = expression.split()
  
      for token in tokens:
          if token.isdigit():
              stack.append(int(token))
          else:
              b = stack.pop()
              a = stack.pop()
              if token == '+':
                  stack.append(a + b)
              elif token == '-':
                  stack.append(a - b)
              elif token == '*':
                  stack.append(a * b)
              elif token == '/':
                  stack.append(a / b)
  
      return stack[0]