input: Write a function that takes a string containing a numerical expression and returns the result of evaluating the expression.
  def eval_expr(expr):
     return eval(expr)
  assert eval_expr("2+2")==4
  assert eval_expr("10 * 5")==50
  assert eval_expr("15 / 3")==5.0
  assert eval_expr("4 ** 2")==16
  assert eval_expr("(2 + 3) * 4")==20
output: def eval_expr(expr):
     return eval(expr)