Add support for "not element of" operator (!∈)

[Don-vip]

I'd like to write this validator rule:

/* Many misuses of building=hangar insteaf of building=barn because it's the same word in French */
*[building=hangar][!aeroway] !∈ *[aeroway] {
  throwWarning: tr("{0} not inside {1}", "{0.tag}", "{2.key}");
}

It's not possible yet, is it possible to add !∈ as a new spatial operator (negation of element of)?