code: public function assertSelectableField($expected, $not, $selector)
      {
          (new FormValueAssertion($this, $selector, $not, $expected))->selectable();
      }
docs: @example
  And check that "User" is selected in "Apply to" select
  And check that "Product(s)" is not selected in "Apply to" select
  
  @Then /^(?:|I )check that "([^"]*)" is(| not) selected in "([^"]*)" select$/