Will #1=f(#1#) mean (#1=f #1#) or #1=(f #1#)?
Since 'f(x) => (quote (f x)), I guess the latter is more
consistent (that is, after seeing #1=, the reader recurses
to gather the longest <datum> and then labels it.)
But I'm not sure the current draft spec is clear enough,
for, according to srfi-38, the "#1=f" part consists a <datum>.