,:y adds a leading unit axis to y ,
giving a result of shape 1,$y .Thus:
$ ,: 2 3 4
1 3
An atomic argument in x,:y is first reshaped to
the shape of the other (or to a list if the other argument
is also atomic); the results are then itemized and catenated,
as in (,:x),(,:y) .
The fit conjunction(,:!.f)provides
fill specified by the atom f ,or
the normal fill defined under Take ({.)
if f is an empty vector.