Effect:

On exit, the value of

   dst [  ]

is

   src [   [  ], ...,
                 [  ]]

unless

   msk [  ]

was zero, in which case the exit value of the dst element is unchanged from the entry value. Here denotest the rth subscript array. If subs [r] is null, the value of the expression [ ] in this formula is defined to be (hence if all subscripts are undefined, this operation is equivalent to a Remap operation).