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HCOLOR=val
Sets the color used for Hires graphics drawing. Valid colors are 0
(Black 1), 1 (Green), 2 (Violet), 3 (White 1), 4 (Black 2), 5
(Orange), 6 (Blue), 7 (White 2).

HGR
Turns on the first hires screen to a mixed 280x160 mode with 4 lines
of text at the bottom, and clears the graphical section to all black.
Does not move the cursor or set a 4-line text window like GR does.

HGR2
Turns on the second hires screen, and clears it to black. It also
displays all of it (280x192) with no text visible at the bottom.
[There's no easy way to get both the second hires screen and 4 lines
of text onscreen in a "mixed" mode as with lores and the first hires
screen without some loops to do memory copying and the like.]

HIMEM:val
Sets the highest memory position available for Applesoft's variables.
Used to reserve some space off for binary program code and the like.
The ':' is part of the command; if you do 'HIMEM=x', you will set the
real variable HI. The himem position is normally set by default; only
mess with it if you know what you're doing; under ProDOS it should be
an even multiple of 1024 (1K).

ProDOS's BASIC.SYSTEM grabs space on the fly for things, so moving
HIMEM under ProDOS may be a risky job. In all cases, it must be a
multiple of 256 under ProDOS.

HLIN x1,x2 AT y
Draws a horizontal line on the lores graphics screen from x1,y to
x2,y. x1 and x2 do not need to be in sorted order. x1, x2, and y
should be within the limits of the lores graphics screen (0..39 for x,
0..47 for y), or the results may be unpredictable.

HOME
Clears the text display screen and positions the cursor in the
top-left. Note that when in Lores mixed mode (40x40+4), the text
screen consists of only 4 lines of text. Use the TEXT command to exit
the graphics mode.

HPLOT [TO] x,y [ TO x1,y1 [ TO x2,y2]]
Draws a point, line, or series of line segments. If only a single
point is specified, it is drawn. With a TO, a line segment is drawn
from the last drawn point to the new location. Use HCOLOR= to set the
color of the line before drawing.

HTAB x
Sets the cursor's horizontal position to x. Under 80-column modes, you
may find that 'POKE 1403,x-1' works more reliably. [Htab uses 1-80 to
set the column; the POKE uses 0-79]

IF expr ...
Expression expr is parsed, and if true (not zero), then the rest of
the line is parsed. If expr is false (logical and numeric zero), then
control skips to the next line of code. No parentheses are needed
around the x clause, but they don't really hurt. There is no else
clause in the language, but it is easy to get around that. 'IF x THEN
GOTO n' can also be written as 'IF x GOTO n' or 'IF x THEN n', but
this abbreviation can only be done for 'THEN GOTO', no other
statements.

Several to all of ProDOS's BASIC.SYSTEM versions went into TRACE mode
if your code had a 'THEN FLASH' evaluated.