Structured flow

z80a also has words that behave similarly to IF..THEN and
BEGIN..UNTIL.

On the IF side, we have IFZ, IFNZ, IFC, IFNC, and THEN,. When
the opposite condition is met, a relative jump is made to
THEN,'s PC. For example, if you have IFZ, a jump is made when
Z is unset.

On the BEGIN,..AGAIN, side, it's a bit different. You start
with your BEGIN, instruction, and then later you issue a
JRxx, instr followed by AGAIN,. Exactly like you would do
with a label.

                                                        (cont.)
