Hey Andre, I believe the line reads slightly differently than you've
interpreted ... some more info (note: I still don't have the answer):

||= is actually a ruby idiom to initialize a variable if it's nil.  In
other words, "a ||= 6" actually means "a = 6 if a.nil?" or "a = 6
unless a".  (sorry if I'm lecturing)  In other words, if @foo really
*is* a value, and it really *is* nil, I think the function call is
going to be made no matter what ... but again, this is a bit over my
head at present.

So, converting __#{id.to_i}__ to something more readable, like "foo",
this: (@foo ||= [foo(*a, &b)] )[ 0 ]

should be the same as the more verbose:

if @foo
  return @foo[ 0 ]   # Still not sure why the [0] is required!
else
  return foo( *a, &b )    # the 0th index of the single array
end

jz