Why is a generator expression called a expression?

[Veek M]

The docs state that a expression is some combination of value, operator, 
variable and function. Also you cannot add or combine a generator 
expression with a value as you would do with 2 + 3 + 4. For example, 
someone on IRC suggested this
all(a == 'a' for a in 'apple') but

1. all is a function/method
2. so (whatever) in this case is a call but obviously it works so it must 
be a generator object as well.. so.. how does 'all' the function object 
work with the generator object that's being produced? 

I can't for example do min 1,2,3 but i can do min (1,2,3) and the () are 
not integral to a tuple - therefore one could argue that the () are part 
of the call - not so with a generator Expression where the () are 
integral to its existence as an object.

Could someone clarify further.