It only became a regular feature, however, beginning with "Invención I" for piano (1958), and subsequently in most of his instrumental compositions of the 1960s and 1970s: "Invención II" for string trio (1965), "Invención III" for harp (1967), "Soli II" for wind quintet (1961), "Soli III" for bassoon, trumpet, viola, timpani, and orchestra (1969), "Soli IV" for brass trio (1966), "Cinco Caprichos" for piano (1975), and the late orchestral works "Resonancias" (1964), "Elatio" (1967), "Discovery" (1969), "Clio" (1969), and "Initium" (1970–72) .