I built DSLINES.EXE in DEVstudio and ran it from the command line using
dslines.exe testfile.for $ DEBUG
MSDEV opened, with testfile.for opened in a window, cursor at the start of the first line. Nothing else happened.
The DOSBOX message, after a while, was
Unable to get ITextSelection object; Aborting; OLE error status = 0x80029C4A; A
MSDEV was left opened after the DOS box closed.
(I had to freeze the box by inserting a 'pause' in the exit routine code in order to save the text)
The testfile.for consists of