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.

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.

*** Defining words ***
: x ...     --              Define a new word
;           R:I --          Exit a colon definition
ALLOT       n --            Move HERE by n bytes
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 --            Inserts number from TOS as a literal
VARIABLE c  --              Creates cell x with 2 bytes allocation.

*** Flow ***
ELSE        --              Branch to THEN
EXECUTE     a --            Execute wordref at addr a
IF          n --            Branch to ELSE or THEN if n is zero
INTERPRET   --              Get a line from stdin, compile it in tmp memory,
                            then execute the compiled contents.
QUIT        R:drop --       Return to interpreter promp immediately
THEN        --              Does nothing. Serves as a branching merker for IF
                            and ELSE.

*** Stack ***
DUP         a -- a a
OVER        a b -- a b a
SWAP        a b -- b a

*** 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
CURRENT     -- n            Set n to wordref of last added entry.
HERE        -- a            Push HERE's address

*** Arithmetic ***

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

*** I/O ***
.           n --            Print n in its decimal form
EMIT        c --            Spit char c to stdout
KEY         -- c            Get char c from stdin