• To extract a sub-array from a matrix apply extractitem to the array of real of the matrix.
  For instance to extract a rectangular sub-array you would specify a step parameter equal to the matrix' ncols.
• To extract rows from a matrix apply extractitem to the matrix itself.
• To extract columns from a matrix apply extractcolumn to the matrix.
  By default, when applied to a matrix, extractcolumn will return one array of real if you extract one column, or one list of arrays of real if you extract more than one column.
• To extract columns from a matrix and get the sub-array as a matrix, specify extractcolumn [...] as matrix.
• To extract columns from a matrix and get the sub-array as a tabulated (tab- and return-delimited) string, specify extractcolumn [...] as string.
• Use the skipping parameter (for instance: skipping 1) to skip the header row(s).