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> @ ;"

*** Native Words ***

: x ... ;   --              Define a new word
.           n --            Print n in its decimal form
@           a -- n          Set n to value at address a
!           n a --          Store n in address a
+           a b -- c        a + b -> c
-           a b -- c        a - b -> c
*           a b -- c        a * b -> c
/           a b -- c        a / b -> c
CREATE x    --              Create cell named x
DOES>       --              See description at top of file
DUP         a -- a a
EMIT        c --            Spit char c to stdout
EXECUTE     a --            Execute word at addr a
EXIT        R:I --          Exit a colon definition
HERE        -- a            Push HERE's address
QUIT        R:drop --       Return to interpreter promp immediately
KEY         -- c            Get char c from stdin
INTERPRET   --              Get a line from stdin, compile it in tmp memory,
                            then execute the compiled contents.
OVER        a b -- a b a
SWAP        a b -- b a

*** Core-but-Forth Words ***

?           a --            Print value of addr a
+!          n a --          Increase value of addr a by n
ALLOT       n --            Move HERE by n bytes
CONSTANT x  n --            Creates cell x that when called pushes its value