This function is intended primarily as a means of iterating round all the columns in a Table. For this purpose, the number of columns in the Table is given by the Ncolumn attribute of the Table. This function could then be called in a loop, with the index value going from zero to one less than Ncolumn.
Note, the index associated with a column decreases monotonically with the age of the column: the oldest Column in the Table will have index one, and the Column added most recently to the Table will have the largest index.
The returned pointer is guaranteed to remain valid and the string to which it points will not be over-written for a total of 50 successive invocations of this function. After this, the memory containing the string may be re-used, so a copy of the string should be made if it is needed for longer than this.
A NULL pointer will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.