- each cell contains a row vector
- the cell can be converted to a matrix. (For example would fail for function handles)
- the elements are a datatype for which sum() is defined
- the size of each row vector is consistent for the column.
- that the elements are numeric
- that the length of the row vectors is exactly 3
- that the length of the row vectors are all the same across the columns. For example if column 2 had row vectors of length 4, then this code would be happy to give you a result in which the second entry had length 4.

