function isCombinator(x) {
  return /[\s+>~]/.test(x);
}
Check whether a selector has an interpolating ampersand
An "interpolating ampersand" is an "&" used to interpolate within another
simple selector (e.g. `&-modifier`), rather than an "&" that stands
on its own as a simple selector (e.g. `& .child`)

@param {string} selector
@return {boolean} If `true`, the selector has an interpolating ampersand