u/.  (Adverb)

OBLIQUE (_)  If a=. i.3 4, then </.a is (,0);1 4;2 5 8;3 6 9;7 10;,11, that is, the (3+4)–1 items of the result are obtained by applying < to each of the oblique lines of y. Hence, if p and q are the coefficients of two polynomials, then +//.p*/q yields the coefficients of their product.

More generally, u/.y is the result of applying u to the oblique lines of _2-cells of y. If the rank of y is less than two, y is treated as the table ;y.

KEY (_ _)  x u/. y is (=x) u@# y, that is, items of x specify keys for corresponding items of y and u is applied to each  collection of y having identical keys. Thus, 1 2 3 1 3 2 1 </.'abcdefg' is 'adg';'bf';'ce' .