I have adopted ASCII marble graphs from The introduction to Reactive Programming you've been missing and will use these here and there to explain how stream operators behave.

S.1: --a---b-c---d---X---|--->

--------->    the timeline
a, b, c, d    emitted items
X             error notification
|             completed notification
T, F          True, False
N             null/nil/undefined

Sources

• The introduction to Reactive Programming you've been missing