Analogy: mathematical expressions

This mechanism is similar to mathematical formulas: a big formula is created by composing small formulas. Such formulas are called expressions, and they can contain other expressions, so you can make arbitrarily complex and large expressions. For example,

1
1 + 2
(1 + 2) * 3
((1 + 2) * 3) / (4 * 5)

This is a sequence of expressions, where each expression is contained in the next (larger) one. The simplest expressions are numbers, and larger ones are made by combining expressions with operators (like +, *, and /) and parentheses. Like mathematical expressions, music expressions can be nested arbitrarily deep, which is necessary for complex music like polyphonic scores.