In reply to Aira:
// The ack[=1] is equivalent to.
!ack[*0:$] ##1 ack ##1 !ack[*0:$] // an infinite sequence
1[*5] // is a sequence that is true (that the "1") for 5 cycles
intersect // is the conjunction where those 2 sequences are common (true)
// so basically, the
intersect 1[*5] // defines the slice where
!ack[*0:$] ##1 ack ##1 !ack[*0:$] // is true.
Thus, the ack[=1] intersect 1[*5]; says there is one ack in 5 cycles.
An English equivalent: from a 100cm tape, look at the slice from 0cm to 5cm.
In harwware design, this is called an evaluation envelope.