ANSI/DEC VT Terminal Character Control Sequences
                    ================================================
I have found that documentation for various VT/ANSI terminal sequences is contradictory
and misleading. After extensive testing, the following sequences are what I use to obtain
these listed effects.

+--------+--------+---------------------+--------------------+----------------------------+
| Char   : Action | VT100 and later     | VT52               : UKNC/VT52                  |
+--------+--------+---------------------+--------------------+----------------------------+
| 07     : Bell   | 07                  | 07                 : 07                         |
+--------+--------+---------------------+--------------------+----------------------------+
| 08     : Left   | 08                  | <esc>D             : <esc>D                     |
| 09     : Right  | <esc>[C             | <esc>C             : <esc>C                     |
| 10     : Down   | <esc>D              | 11                 : 10                         |
| 11     : Up     | <esc>M              | <esc>I             : <esc>I                     |
| 12     : CLS    | <esc>[2J<esc>[1;1H  | <esc>H<esc>J       : <esc>H<esc>J               |
| 13     : CR     | 13                  | 13                 : 13                         |
| 30     : Home   | <esc>[1;1H          | <esc>Y<spc><spc>   : <esc>Y<spc><spc>           |
| 31,x,y : TAB    | <esc>[1+y;1+xH      | <esc>Y<32+y><32+x> : <esc>Y<32+y><32+x>         |
| 127    : Delete | 08<spc>08           | 08<spc>08          : 08<spc>08                  |
+--------+--------+---------------------+--------------------+----------------------------+
| 17,n   : COLOUR | <esc>[...m          |                    : &00+n: <esc>,160,'0'-'7'   |
|        :        |                     |                    : &40+n: <esc>,167,'0'-'7'   |
|        :        |                     |                    : &80+n: <esc>,161,'0'-'7',  |
|        :        |                     |                    :        <esc>,162,'0'-'7'   |
| 20     : Reset  | <esc>[0m            |                    :<esc>,191,163,<esc>,191,164,|
|        :        |                     |                    :<esc>,160,'7',<esc>,167,'7',|
|        :        |                     |                    :<esc>,161,'0',<esc>,162,'0' |
| 22,n   : MODE   | as VDU 20,12        |                    :<esc>,166,'1'-'4'           |
|        :        |                     |                    :      then VDU 20,12        |
+--------+--------+---------------------+--------------------+----------------------------+
| 16     : CLG    |                     |                    :                            |
| 18,a,n : GCOL   |                     |                    :                            |
| 19...  : Palette|                     |                    :                            |
| 23...  :        |                     |                    :                            |
| 24...  :        |                     |                    :                            |
| 25...  : PLOT   |                     |                    :                            |
| 26...  :        |                     |                    :                            |
| 28...  :        |                     |                    :                            |
| 29...  :        |                     |                    :                            |
+--------+--------+---------------------+--------------------+----------------------------+

Notes
-----
08 and 13 appear to be the only universal control characters. Everything else appears to
be interpreted differently by different systems. In particular, the VT100 appears to
implement 10 as NEWLINE instead of DOWN, necessitating using <esc>D - which is different
to <esc>D on the VT52!

Documentation and testing shows that <esc>A/B/C/D move the cursor, but fail to scroll when
at the edge of the screen, resulting in text being overwritten. This neccessitates using
using <esc>D and <esc>M on the VT100 and 10 or 11 and <esc>I on the VT52.

Even character 08 can't be relied on. The VT52 can be switched to a different mode where 08
is Home not Left, so <esc>D has to be used.

ANSI colour selection builds an <esc>[0;7;4;5;30;1;40;100m sequence as follows:

Initialise with:
  fgd=7:bgd=0:PRINT CHR$27;"[0,";                :REM White on black

Select colour with:
  IF (C >= 192):border=C AND 15:ENDPROC          :REM Set border colour
  IF (C AND 128):bgd=C AND 15 ELSE fgd=C AND 15  :REM Set new colour
  PRINT CHR$27;"[0";
  IF (C AND 16):PRINT ";5";                      :REM Flash
  IF (C AND 32):PRINT ";4";                      :REM Underline
  IF (C AND 64):PRINT ";7";                      :REM Inverse
  PRINT ";";30+(fgd AND 7);                      :REM Foreground colour
  IF (fgd AND 8):PRINT ";1";                     :REM Bright foreground
  PRINT ";";40+(bgd AND 7);                      :REM Background colour
  IF (bgd AND 8):PRINT ";";100+(bgd AND 7);      :REM Bright background
  PRINT "m";
  
On some platforms [21m is not the opposite of [1m, so they cannot be used as a pair,
so a reset sequence has to be sent instead.

On the UKNC the RGB colour number 0,1,2,3,4,5,6,7 is mapped to 0,4,2,6,1,5,3,7.

RT11Em's VT52 implementation appears to have bugs in it. CHR$10 does NEWLINE instead of
DOWN, but CHR$11 does DOWN. A VT100 <esc>[...m colour sequence appears to crash either
RT11 or the emulator.