Generic Function stream-start-line-p
This is a predicate which returns t if the stream is positioned at the beginning of a line, else nil. It is permissible to always return nil. This is used in the implementation of cl:fresh-line. Note that while a value of 0 from stream-line-column also indicates the beginning of a line, there are cases where stream-start-line-p can be meaningfully implemented although stream-line-column can't be. For example, for a window using variable-width characters, the column number isn't very meaningful, but the beginning of the line does have a clear meaning. The default method for stream-start-line-p on class fundamental-character-output-stream uses stream-line-column, so if that is defined to return nil, then a method should be provided for either stream-start-line-p or stream-fresh-line.