+This implementation checks for three cases:
+.RS 3
+.bP
+the parameter is a 7-bit US\-ASCII code.
+This is the case that X/Open Curses documented.
+.bP
+the parameter is in the range 128\-159, i.e., a C1 control code.
+If \fBuse_legacy_coding\fP has been called with a \fB2\fP parameter,
+\fBunctrl\fP returns the parameter, i.e., a one-character string with
+the parameter as the first character.
+Otherwise, it returns ``~@'', ``~A'', etc., analogous to ``^@'', ``^A'', C0 controls.
+.IP
+X/Open Curses does not document whether \fBunctrl\fP can be called before
+initializing curses.
+This implementation permits that,
+and returns the ``~@'', etc., values in that case.
+.bP
+parameter values outside the 0 to 255 range.
+\fBunctrl\fP returns a null pointer.
+.RE