code: function unset (selector, prop) {
      if (prop !== undefined) {
        delete rules[selector][prop]
      } else {
        delete rules[selector]
      }
      return api
    }
docs: .unset() removes a ruleset from the sistyl instance, that corresponds to the given selector. Note that it removes *just* the given selector, and not other rulesets that also match the selector. Specifically, .unset('.rem') does *not* remove a '.keep, .rem' selector.  style.unset('.selector') // removes the `.selector {}` // ruleset style.unset('.selector', // removes the `color` property 'color')     // from the `.selector` ruleset.