.pg
.x if
.x then
.x elseif
.x else
.x end
.lt
------------------------------------------------------------------------
IF [NOT] predicate THEN
  ...
[ELSEIF [NOT] predicate] THEN
  ...
[ELSE]
  ...
END
------------------------------------------------------------------------
.el
.b 3
The IF macro generates code to execute a alternative control structure.
.b
The ^&predicate\& may take one of the following forms:
.ls
.le;condition
.le;instruction condition
.le;instruction ... condition
.els
where ^&condition\& is one of the following mnemonics:
.b
.x if > eql
.x if > eqlu
.x if > neq
.x if > nequ
.x if > gtr
.x if > gtru
.x if > lss
.x if > lssu
.x if > geq
.x if > gequ
.x if > leq
.x if > lequ
.x if > cc
.x if > cs
.x if > vc
.x if > vs
.x if > lbs
.x if > lbc
.x if > error
.x if > ok
.x if > even
.x if > odd
.x if > bs
.x if > bc
.x if > bss
.x if > bcs
.x if > bsc
.x if > bcc
.lt
	EQL	EQLU		NEQ	NEQU
	GTR	GTRU		LSS	LSSU
	GEQ	GEQU		LEQ	LEQU

	CC	CS		VC	VS

	<LBS source>		OK	ODD
	<LBC source>		ERROR	EVEN

	<BS  position,base>	<BC  position,base>

	<BSS position,base>	<BCS position,base>
	<BSC position,base>	<BCC position,base>
.el
.b
and ^&instruction\& can be any assembly instruction.
.b
.x not
.x if > not
If NOT is specified, all ^&conditions\& in the predicate are negated.
.b
.x and
.x if > and
.x or
.x if > or
Compound predicates may be formed by using the boolean operators AND and OR.
Note that the operators associate to the right, so that the implied grouping of
"A OR B AND C OR D" is "A OR (B AND (C OR D))". 
.b
.x andif
.x if > andif
.x orif
.x if > orif
In order to associate to the left, we use the operators ANDIF and ORIF. In
other words, ANDIF and ORIF act as a closing parenthesis for the expression to
their left and as an opening parenthesis for the expression to their right;
hence, "A OR B ANDIF C OR D" is "(A OR B) AND (C OR D)". 
.b
The ELSEIF macro ends the true part of an IF structure and begins a subsequent
nested IF structure. The ELSEIF macro is optional and can be specified as often
as required.
.b
The ELSE macro ends the true part of an IF structure and begins the false part.
The ELSE macro is optional.
.b
The default branch displacment for IF is WORD (see BRANCH).
.b
.tp 5
The following IF structure:
.b
.lt
	if <tstw number> lss then
		statements-1
	elseif gtr then
		statements-2
	elseif eql then
		statements-3
	end
.el
.b
is equivalent to:
.b
.lt
	if <tstw number> lss then
		statements-1
	else
		if gtr then
			statements-2
		else
			if eql then
				statements-3
			end
		end
	end
.el