Regarding Callas's technical prowess, Celletti says, "We must not forget that she could tackle the whole gamut of ornamentation: staccato, trills, half-trills, gruppetti, scales, etc." D'Amico adds, "The essential virtue of Callas's technique consists of supreme mastery of an extraordinarily rich range of tone colour (that is, the fusion of dynamic range and timbre). And such mastery means total freedom of choice in its use: not being a slave to one's abilities, but rather, being able to use them at will as a means to an end." While reviewing the many recorded versions of "perhaps Verdi's ultimate challenge", the aria "D'amor sull'ali rosee" from "Il trovatore", Richard Dyer writes,