4E. triple (IF THEN ELSE)
-------------------------
The IF operator has quite the same function as the IF statement, only
it selects between two expressions instead of two statements or blocks
of statements. It equals the x?y:z operator in C.

IF <boolexp> THEN <exp1> ELSE <exp2>

returns exp1 or exp2, according to boolexp. For example, instead of:

IF a<1 THEN b:=2 ELSE b:=3
IF x=3 THEN WriteF('x is 3\n') ELSE WriteF('x is something else\n')

write:

b:=IF a<1 THEN 2 ELSE 3
WriteF(IF x=3 THEN 'x is 3\n' ELSE 'x is something else\n')