What's an A-expression? I tried looking it up but didn't find anything. I have heard of M-expressions but never of an actual implementation. Scheme also seems

What's an A-expression? I tried looking it up but didn't find anything. I have heard of M-expressions but never of an actual implementation. Scheme also seems to have some SRFIs involving alternate syntax that's whitespace dependent, like SRFI 119 (wisp), SRFI 110 (sweet-expressions or T-expressions), or SRFI 49.