This function calculates the moving-average type-token ratio (MATTR). In contrast to
lex.div,
which by default calculates all possible measures and
their progressing characteristics, this function will only calculate the MATTR value.
References
Covington, M.A. & McFall,
J.D. (2010). Cutting the Gordian Knot: The Moving-Average Type-Token Ratio (MATTR).
Journal of Quantitative Linguistics, 17(2), 94--100.