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')


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