Return the value of comment identified by name from the image store specified by handle. If retvar is specified the command returns `1' if the comment is found and `0' if it is not found. The value of the comment is returned in the variable retvar, unless retvar is `{}', in which the only the presence of the variable is returned, but not value. If retvar is not specified, the command returns the value of the comment or an empty string if the comment has no value or is not. If no arguments are specified, then a keyed list of all comments is returned.
If the image store does not support comments, a error is returned.