We have constructed two types of boundaries that are void-equivalent whenever they form a double-boundary.
A **double-boundary** is a structure in which one boundary is the sole contents of another boundary form. A **simply nested form** incorporates no container with more than one content form.
The round/square pair of boundaries are jointly called an **inversion pair** when there are no intervening forms. Inversion pairs are void-equivalent and can be freely deleted. **They are structural noise within an intentional signal.** Due to Inversion, the only simply nested forms that do not reduce consist of purely either round- or square-brackets. Simply nested forms will take on greater structural variety after we introduce the angle-bracket in Chapter 10.
Inversion boundaries are void-equivalent only when they are simply nested. Should the outer boundary contain other contents, the pair of boundaries do not reduce.
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BRICKEN, William, 2019. Iconic Arithmetic Volume I: The Design of Mathematics for Human Understanding. Unary press. ISBN 978-1-73248-513-6, p. 185.