constructor action/Lambda --then/Lambda?=null:
foo --then=(::print foo) --then=(::print bar)
awaitAll
Promise (:: print "Hello")
.then (:: print "Hello again")
Promise (:: print "Hello 1")
.then (:: print "Hello 2")
.then (:: print "Hello 3")
.then (:: print "Hello 4")
.await
Promise [
:: print "Hello 1",
:: print "Hello 2",
...
]