enter_underline_mode
enter_underline_mode = ABSENT_STRING;
&& VALID_STRING(enter_underline_mode))
|| PRESENT(enter_underline_mode)
PAIRED(enter_underline_mode, exit_underline_mode);
if (enter_underline_mode)
enter_underline_mode = ABSENT_STRING;
TurnOn(A_UNDERLINE, enter_underline_mode);
if (enter_underline_mode)
TurnOn(A_UNDERLINE, enter_underline_mode);
tmodes(enter_underline_mode, exit_underline_mode, &sc_u_in, &sc_u_out,
CHECK_SGR(2, enter_underline_mode);