Find the line in the present routine referenced
by the lineref in Result.
If no such line exists, place
the null string ("") into C.
Otherwise, line C.
If Result < 0, trap execution.
Otherwise, Result B.
If B = 0, Present routine name
C.
Otherwise, find the Bth
line of the present routine.
If no such line
exists, place the null string ("") into C.
Otherwise, line C.
If C is not the null string (""), and C is not the Present
routine name, replace the ls in C with one SP (space) and remove
the eol.
C Result.
C.