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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>.