lessThan
fun ObservableDouble.lessThan(other: ObservableDouble): BooleanBinaryFunction<Double, ObservableDouble, Double, ObservableDouble>
fun ObservableFloat.lessThan(other: ObservableFloat): BooleanBinaryFunction<Float, ObservableFloat, Float, ObservableFloat>
fun ObservableInt.lessThan(other: ObservableInt): BooleanBinaryFunction<Int, ObservableInt, Int, ObservableInt>
fun ObservableLong.lessThan(other: ObservableLong): BooleanBinaryFunction<Long, ObservableLong, Long, ObservableLong>