function clearOverrides(overrides) {
  // create mutable overrides
  overrides = NSMutableDictionary.dictionaryWithDictionary(overrides)

  // filter dictionary
  let keys = overrides.allKeys()
  keys.forEach(key => {
    let value = overrides.objectForKey(key)
    if (value.isKindOfClass(NSDictionary.class())) {
      value = clearOverrides(value)
      if (value.allKeys().count() > 0) {
        overrides.setValue_forKey(value, key)
      } else {
        overrides.removeObjectForKey(key)
      }
    } else {
      if (key !== 'symbolID' && String(key).indexOf('-original') === -1) {
        overrides.removeObjectForKey(key)
      }
    }
  })

  // restore original overrides
  keys = overrides.allKeys()
  keys.forEach(key => {
    if (String(key).indexOf('-original') > -1) {
      let value = overrides.objectForKey(key)
      overrides.removeObjectForKey(key)
      overrides.setValue_forKey(value, String(key).replace('-original', ''))
    }
  })

  return overrides
}
Removes all 'content' data from overrides, keeping only symbol overrides.

@param {NSDictionary} overrides
@returns {NSDictionary}