Use of `D' or `d' as the first character (column 1) of a source line denotes a debug line.
In turn, a debug line is treated as either a comment line or a normal line, depending on whether debug lines are enabled.
When treated as a comment line, a line beginning with `D' or `d' is treated as if it the first character was `C' or `c', respectively. When treated as a normal line, such a line is treated as if the first character was <SPC> (space).
(Currently, g77 provides no means for treating debug lines as normal lines.)