idlwave-begin-line-comment

idlwave-begin-line-comment is a variable defined in `idlwave.el'.
Its value is

nil

Documentation:
A comment anchored at the beginning of line.
A comment matching this regular expression will not have its
indentation changed. If nil the default is "^;", i.e., any line
beginning with a ";". Expressions for comments at the beginning of
the line should begin with "^".

You can customize this variable.