English
The word of a product equals the reduction of the concatenation of the words: toWord(x*y) = reduce (toWord x ++ toWord y).
Русский
Слово произведения равно редукции конкатенации слов: toWord(x*y) = reduce (toWord x ++ toWord y).
LaTeX
$$$\forall x,y : FreeGroup\,\alpha ,\; toWord(x * y) = \mathrm{reduce}(toWord(x) ++ toWord(y))$$$
Lean4
@[to_additive]
theorem toWord_mul (x y : FreeGroup α) : toWord (x * y) = reduce (toWord x ++ toWord y) :=
by
rw [← mk_toWord (x := x), ← mk_toWord (x := y)]
simp