Confusion about buildEither of result builder

Using Swift
1

I'm learning result builder. This part of the doc
confuses me:
--- Quote Start ---

  • A branch statement becomes a series of nested calls to the buildEither(first:) and buildEither(second:)methods. The statements’ conditions and cases are mapped onto the leaf nodes of a binary tree, and the statement becomes a nested call to the buildEither methods following the path to that leaf node from the root node.For example, if you write a switch statement that has three cases, the compiler uses a binary tree with three leaf nodes. Likewise, because the path from the root node to the second case is “second child” and then “first child”, that case becomes a nested call like buildEither(first: buildEither(second: ... )). The following declarations are equivalent:
let someNumber = 19
@ArrayBuilder var builderConditional: [Int] {
    if someNumber < 12 {
        31
    } else if someNumber == 19 {
        32
    } else {
        33
    }
}


var manualConditional: [Int]
if someNumber < 12 {
    let partialResult = ArrayBuilder.buildExpression(31)
    let outerPartialResult = ArrayBuilder.buildEither(first: partialResult)
    manualConditional = ArrayBuilder.buildEither(first: outerPartialResult)
} else if someNumber == 19 {
    let partialResult = ArrayBuilder.buildExpression(32)
    let outerPartialResult = ArrayBuilder.buildEither(second: partialResult)
    manualConditional = ArrayBuilder.buildEither(first: outerPartialResult)
} else {
    let partialResult = ArrayBuilder.buildExpression(33)
    manualConditional = ArrayBuilder.buildEither(second: partialResult)
}

--- Quote End ---

I thought

    if someNumber < 12 {
        31
    } else if someNumber == 19 {
        32
    } else {
        33
    }

is interpreted as

if someNumber < 12 {
    31
} else {
    if someNumber == 19 {
        32
    } else {
        33
    }
}

and it should be

var manualConditional: [Int]
if someNumber < 12 {
    let partialResult = ArrayBuilder.buildExpression(31)
    let manualConditional = ArrayBuilder.buildEither(first: partialResult)
} else if someNumber == 19 {
    let partialResult = ArrayBuilder.buildExpression(32)
    let outerPartialResult = ArrayBuilder.buildEither(first: partialResult)
    manualConditional = ArrayBuilder.buildEither(second: outerPartialResult)
} else {
    let partialResult = ArrayBuilder.buildExpression(33)
    let outerPartialResult = ArrayBuilder.buildEither(second: partialResult)
    manualConditional = ArrayBuilder.buildEither(second: outerPartialResult)
}

but it seems it is interpreted as

if what-goes-here {
    if someNumber < 12 {
        31
    } else if someNumber == 19 {
        32
    } 
} else {
    33
}

which doesn't seem right to me.

2

The proposal says it builds a "balanced binary tree", which necessitates such a transformation if there are four or more total branches -- if an if-else chain were translated naively, it wouldn't form a balanced tree. The fact that it does this for exactly three branches seems to be, as far as I can tell, an implementation detail that doesn't strictly matter.

3

Also “balanced binary tree” is underspecified, because for example AVL trees and red black trees use a different definition of “balanced”. Perhaps the authors inserted the word “balanced” on accident when they just meant to say “binary tree”.

4

Huh, TIL. AVL trees were never brought up in any data structures course I've taken.