u} y Composite Item Adverb
u} y executes u y to calculate the item indexes m to be used at each atom, and then produces the result m} y .
The construct u} y is an obsolete relic of early J, superseded by the gerund form of Composite Items (m} y).
x u} y Amend Adverb
x u} y executes x u y to calculate the atom numbers to be modified, and then replaces those atoms of y with atoms of x.
The atom number of the atom of y whose index list is d is ($y)#.d .
The construct x u} y is an obsolete relic of early J, superseded by the gerund form of Amend (x m} y).