def str_index(string, substring)
      assert_type string, :String, :string
      assert_type substring, :String, :substring
      index = string.value.index(substring.value)
      index ? number(index + 1) : null
    end
def str_index(string, substring)
      assert_type string, :String, :string
      assert_type substring, :String, :substring
      index = string.value.index(substring.value)
      index ? number(index + 1) : null
    end
Returns the index of the first occurrence of `$substring` in `$string`. If
 there is no such occurrence, returns `null`.

 Note that unlike some languages, the first character in a Sass string is
 number 1, the second number 2, and so forth.

 @example
   str-index(abcd, a)  => 1
   str-index(abcd, ab) => 1
   str-index(abcd, X)  => null
   str-index(abcd, c)  => 3

 @overload str_index($string, $substring)
   @param $string [Sass::Script::Value::String]
   @param $substring [Sass::Script::Value::String]
 @return [Sass::Script::Value::Number, Sass::Script::Value::Null]
 @raise [ArgumentError] if any parameter is the wrong type