+The outermost parentheses were added because we have an application. `(\x. \y.
+...)` became `(\x (\y. ...))` because of the rule for dots. We didn't
+insert any parentheses around the inner body of `\y. (x y)` because they were
+already there. That is, in expressions of the form `\y. (...)`, the dot abbreviates
+nothing. It's harmless to write such a dot, though, and it can be conceptually
+helpful especially in light of the next convention.
+
+Similarly, we permit `\x. x`, which is shorthand for `\x x`, not for `\x (x)`, which
+our syntax forbids. (The [[lambda evaluator|/code/lambda_evaluator]] however tolerates such expressions.)