/// Checks for a valid number.
///
/// @param {Number} $value
///
/// @require {function} contains

@function is-number($value) {
  @return contains("0" "1" "2" "3" "4" "5" "6" "7" "8" "9" 0 1 2 3 4 5 6 7 8 9, $value);
}
