fastest way to use GContracts class invariant in groovyConsole

@Grab(group='org.gcontracts', module='gcontracts', version='[1.1.0,)')
import org.gcontracts.annotations.Invariant

@Invariant({ firstName != null && lastName != null })
class Person {
  String firstName
  String lastName
  
  def Person(final String firstName, final String lastName)  {
    this.firstName = firstName
    this.lastName = lastName
  }
  
  def String toString ()  { firstName + ' ' + lastName }
}