- These functions are not part of the XSI interface. Some other curses
- implementations are known to have similar, undocumented features, but
- they are not compatible with ncurses.
+ These functions are not part of the X/Open Curses interface. Some
+ other curses implementations are known to have similar features, but
+ they are not compatible with <EM>ncurses</EM>:
+
+ <STRONG>o</STRONG> SVr4 provided <STRONG>traceon</STRONG> and <STRONG>traceoff</STRONG>, to control whether debugging
+ information was written to the "trace" file. While the functions
+ were always available, this feature was only enabled if <STRONG>DEBUG</STRONG> was
+ defined when building the library.
+
+ The SVr4 tracing feature is undocumented.
+
+ <STRONG>o</STRONG> PDCurses provides <STRONG>traceon</STRONG> and <STRONG>traceoff</STRONG>, which (like SVr4) are
+ always available, and enable tracing to the "trace" file only when
+ a debug-library is built.
+
+ PDCurses has a short description of these functions, with a note
+ that they are not present in X/Open Curses, <EM>ncurses</EM> or NetBSD. It
+ does not mention SVr4, but the functions' inclusion in a header
+ file section labeled "Quasi-standard" hints at the origin.