@@ -190,6 +190,35 @@ This is the preferred way to write an expression on a single line. The rune its
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191191Nearly all rune expressions can be written in either form, but there are exceptions. `|%` and `|_` expressions, for example, can only be written in tall form. (Those are a bit too complicated to fit comfortably on one line anyway.)
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193+#### Nesting Runes
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195+Since runes take a fixed number of children, one can visualize how Hoon expressions are built by thinking of each rune being followed by a series of boxes to be filled - one for each of its children. Let us illustrate this with the `:-` rune.
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199+Here we have drawn the `:-` rune followed by a box for each of its two children. We can fill these boxes with either a value or an additional rune. The following figure corresponds to the Hoon expression `:- 2 3`.
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203+This, of course, evaluates to the cell `[2 3]`. This next figure corresponds to the Hoon expression `:- :- 2 3 4`.
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207+This evaluates to `[[2 3] 4]`, and we can think of the second `:-` as being "nested" inside of the first `:-`.
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209+What Hoon expression does the following figure correspond to, and what does it evaluate to?
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213+Right. This represents the Hoon expression `:- 2 :- 3 4`,
214+and evaluates to `[2 [3 4]]`. Remember, though, that if you input this into dojo it will print as `[2 3 4]`.
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216+Thinking in terms of these "LEGO block" diagrams, as well as the more literal binary tree diagrams utilized in [Lesson 1.2](@/docs/tutorials/hoon/nouns.md), can be a helpful learning and debugging tactic.
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218+##### Exercise 1.3.a.
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220+Draw binary trees corresponding to the above figures. Determine a one-to-one correspondence between linearly ordered nested boxes (i.e. what is depicted in the above figures) and binary trees. You will need to make use of the convention `[a [b c]] = [a b c]`.
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193222#### Irregular Forms
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195224Some runes are used so frequently that they have irregular counterparts that are easier to write and which mean precisely the same thing. Irregular rune syntax is hence a form of [syntactic sugar](https://en.wikipedia.org/wiki/Syntactic_sugar). All irregular rune syntax is flat. (It follows from this that all tall form expressions are regular.)