Re-arrange source code and tests

Sources/SwiftUIMath/Internal/Syntax/Atoms/Inner.swift

@@ -0,0 +1,68 @@
+import Foundation
+
+extension Math {
+  final class Inner: Atom {
+    var innerList: AtomList?
+
+    var leftBoundary: Atom? {
+      didSet {
+        if let leftBoundary, leftBoundary.type != .boundary {
+          assertionFailure("Left boundary must be of type 'boundary'")
+          self.leftBoundary = nil
+        }
+      }
+    }
+
+    var rightBoundary: Atom? {
+      didSet {
+        if let rightBoundary, rightBoundary.type != .boundary {
+          assertionFailure("Right boundary must be of type 'boundary'")
+          self.rightBoundary = nil
+        }
+      }
+    }
+
+    override var description: String {
+      [
+        "\\inner",
+        leftBoundary.map { "[\($0.nucleus)]" },
+        innerList.map { "{\($0)}" },
+        rightBoundary.map { "[\($0.nucleus)]" },
+        superscript.map { "^{\($0)}" },
+        `subscript`.map { "_{\($0)}" },
+      ]
+      .compactMap(\.self)
+      .joined()
+    }
+
+    override var finalized: Math.Atom {
+      let finalized = super.finalized
+
+      if let inner = finalized as? Inner {
+        inner.innerList = inner.innerList?.finalized
+      }
+
+      return finalized
+    }
+
+    init(_ inner: Inner) {
+      self.innerList = inner.innerList.map { AtomList($0) }
+      self.leftBoundary = inner.leftBoundary.map { $0.copy() }
+      self.rightBoundary = inner.rightBoundary.map { $0.copy() }
+
+      super.init(inner)
+    }
+
+    init(
+      innerList: AtomList? = nil,
+      leftBoundary: Atom? = nil,
+      rightBoundary: Atom? = nil
+    ) {
+      self.innerList = innerList
+      self.leftBoundary = leftBoundary
+      self.rightBoundary = rightBoundary
+
+      super.init(type: .inner)
+    }
+  }
+}