... They allow the user to specify a frame inside (or in some case outside) the subset to work on. This is done with subroutine GDSBOX which returns two arrays containing the grid coordinates of the frame. ... If a task produces output data, it should write BLANKs (undefined values) outside the frame returned by GDSBOX. ... Further arguments to INITPTR are the dimension of the subset, the grid coordinates of the edges of the subsets and the grid coordinates returned from GDSBOX. ...