Stack notation: "<stack before> -- <stack after>". Rightmost is top of stack
(TOS). For example, in "a b -- c d", b is TOS before, d is TOS after. "R:" means
that the Return Stack is modified. "I:" prefix means "IMMEDIATE", that is, that
this stack transformation is made at compile time.

DOES>: Used inside a colon definition that itself uses CREATE, DOES> transforms
that newly created word into a "does cell", that is, a regular cell ( when
called, puts the cell's addr on PS), but right after that, it executes words
that appear after the DOES>.

"does cells" always allocate 4 bytes (2 for the cell, 2 for the DOES> link) and
there is no need for ALLOT in colon definition.

At compile time, colon definition stops processing words when reaching the
DOES>.

Example: ": CONSTANT CREATE HERE @ ! DOES> @ ;"

Word references (wordref): When we say we have a "word reference", it's a
pointer to a words *code link*. For example, the label "PLUS:" in this unit is a
word reference. Why not refer to the beginning of the word struct? Because we
actually seldom refer to the name and prev link, except during compilation, so
defining "word reference" this way makes the code easier to understand.

Atom: A word of the type compiledWord contains, in its PF, a list of what we
call "atoms". Those atoms are most of the time word references, but they can
also be references to NUMBER and LIT.

Words between "()" are "support words" that aren't really meant to be used
directly, but as part of another word.

"*I*" in description indicates an IMMEDIATE word.

*** Defining words ***
: x ...     --              Define a new word
;           R:I --          Exit a colon definition
,           n --            Write n in HERE and advance it.
' x         -- a            Push addr of word x to a.
['] x       --              *I* Like "'", but spits the addr as a number
                            literal.
(           --              *I* Comment. Ignore rest of line until ")" is read.
ALLOT       n --            Move HERE by n bytes
C,          b --            Write byte b in HERE and advance it.
CREATE x    --              Create cell named x. Doesn't allocate a PF.
CONSTANT x  n --            Creates cell x that when called pushes its value
DOES>       --              See description at top of file
IMMEDIATE   --              Flag the latest defined word as immediate.
LITERAL     n --            *I* Inserts number from TOS as a literal
VARIABLE c  --              Creates cell x with 2 bytes allocation.

*** Flow ***
(fbr)       --              Branches forward by the number specified in its
                            atom's cell.
ELSE        I:a --          *I* Compiles a (fbr) and set branching cell at a.
EXECUTE     a --            Execute wordref at addr a
IF          -- I:a          *I* Compiles a (fbr?) and pushes its cell's addr
INTERPRET   --              Get a line from stdin, compile it in tmp memory,
                            then execute the compiled contents.
QUIT        R:drop --       Return to interpreter promp immediately
RECURSE     R:I -- R:I-2    Run the current word again.
SKIP?       f --            If f is true, skip the execution of the next atom.
                            Use this right before ";" and you're gonna have a
                            bad time.
THEN        I:a --          *I* Set branching cell at a.

*** Parameter Stack ***
DUP         a -- a a
OVER        a b -- a b a
SWAP        a b -- b a
2DUP        a b -- a b a b
2OVER       a b c d -- a b c d a b
2SWAP       a b c d -- c d a b

*** Return Stack ***
>R          n -- R:n        Pops PS and push to RS
R>          R:n -- n        Pops RS and push to PS
I           -- n            Copy RS TOS to PS
I'          -- n            Copy RS second item to PS
J           -- n            Copy RS third item to PS

*** Memory ***
@           a -- n          Set n to value at address a
!           n a --          Store n in address a
?           a --            Print value of addr a
+!          n a --          Increase value of addr a by n
C@          a -- c          Set c to byte at address a
C!          c a --          Store byte c in address a
CURRENT     -- a            Set a to wordref of last added entry.
HERE        -- a            Push HERE's address
H           -- a            HERE @

*** Arithmetic ***

+           a b -- c        a + b -> c
-           a b -- c        a - b -> c
-^          a b -- c        b - a -> c
*           a b -- c        a * b -> c
/           a b -- c        a / b -> c

*** Logic ***
=           n1 n2 -- f      Push true if n1 == n2
<           n1 n2 -- f      Push true if n1 < n2
>           n1 n2 -- f      Push true if n1 > n2
CMP         n1 n2 -- n      Compare n1 and n2 and set n to -1, 0, or 1.
                            n=0: a1=a2. n=1: a1>a2. n=-1: a1<a2.
NOT         f -- f          Push the logical opposite of f

*** Strings ***
LIT@ x      -- a            Read following LIT and push its addr to a
SCMP        a1 a2 -- n      Compare strings a1 and a2. See CMP

*** I/O ***
.           n --            Print n in its decimal form
EMIT        c --            Spit char c to stdout
KEY         -- c            Get char c from stdin
PC!         c a --          Spit c to port a
PC@         a -- c          Fetch c from port a
WORD        -- a            Read one word from stdin and push its addr