I think you are a:
Because each line refers to one or more member functions of a C++ std::map.
The sky is so clear
As if I am in emptiness.
"Insert coin here"
Says the arcade just emplaced.
I will bring my eraser
If my paper is swapped with a scrap.
Nothing to extract from the scrap
Since they'll merge together anyway.
I count and find
count() and find()
The supremum and the infimum.
These are terms for particular elements of a set, in this case the keyset of the map, which are related to upper_bound() and lower_bound()
At the square brackets
The square brackets operator  looks up an element of a map, as does the at() function.
Thanks to OP and Gareth for correcting me on this, because I originally answered "set" but of course you can't look up anything in a set using  or at().
I begin and end.
begin() and end()