test1: ---
assert(IntPredicate { it < 0 }.accept(a))
                               |      |
                               |      1
                               false
---
test2: ---
assert(IntPredicate { it < 0 }.accept(a) == true)
                               |      |  |
                               |      |  false
                               |      1
                               false
---
test3: ---
assert(IntPredicate { it > 0 } plus a)
                               |    |
                               |    -1
                               false
---
