Is STREAM known to be positioned at the beginning of a line?
It is permissible for an implementation to always return
NIL. This is used in the implementation of 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.