English
The length function on FreeAddMagma is defined by length(of x) = 1 and length(u + v) = length(u) + length(v).
Русский
Функция длины на FreeAddMagma задаётся так: длина(of x) = 1 и длина(u + v) = длина(u) + длина(v).
LaTeX
$$$\operatorname{length} : \mathrm{FreeAddMagma} \alpha \to \mathbb{N},\quad \operatorname{length}(\mathrm{of}(x)) = 1,\quad \operatorname{length}(u + v) = \operatorname{length}(u) + \operatorname{length}(v)$$$
Lean4
/-- Length of an element of a free additive magma. -/
def length {α : Type u} : FreeAddMagma α → ℕ
| FreeAddMagma.of _x => 1
| x + y => x.length + y.length