Why can't we use Drop[N]#Head ? We would get a type error

    [error] /Users/chris/private/d/work/slick/slick-testkit/src/main/scala/com/typesafe/slick/testkit/tests/MapperTest.scala:305: value unary_! is not a member of h.Apply[scala.slick.collection.heterogenous.Succ[scala.slick.collection.heterogenous.Zero.type]]
    [error]       .filter { h => !h(1) }

but why?