The generic function stream-start-line-p returns t if stream is positioned at the beginning of a line, and nil otherwise. It is permissible to define a method that always returns nil.
Note that although 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 and stream-line-column cannot. For example, for a window using variable-width characters the column number is not very meaningful, whereas the beginning of a line has a clear meaning.
The default method for stream-start-line-p on class fundamental-character-output-stream uses stream-line-column. Therefore, if this is defined to return nil, a method should be provided for either stream-start-line-p or stream-fresh-line.
LispWorks User Guide and Reference Manual - 20 Sep 2017