Flow

Note that flow words can only be used in definitions. In the
INTERPRET loop, they don't have the desired effect because each
word from the input stream is executed immediately. In this
context, branching doesn't work.

f IF A ELSE B THEN: if f is true, execute A, if false, execute
B. ELSE is optional.
BEGIN .. f UNTIL: if f is false, branch to BEGIN.
BEGIN .. AGAIN: Always branch to BEGIN.
x y DO .. LOOP: LOOP increments y. if y != x, branch to DO.
x CASE y OF A ENDOF z OF B ENDOF C ENDCASE: If x == y, execute
A, if x == z, execute B. Otherwise, execute C. x is dropped
in case of an OF match, *but it is kept if it reaches C*. You
have to consume it to avoid PSP leak.                   (cont.)