Jon Claerbout 1998 writes: ``*The simplest and most fundamental
linear operators arise when a matrix operator reduces to a simple row
or column*.'' A row is a summation operation and a column is an
impulse response. If the inner loop of a matrix-multiply ranges within
a *row*, the operator is called *sum* or *pull*.
If the inner loop ranges
within a *column*, the operator is called *spray* or *push*.

7/5/1998