The logical operators begin and end with a full stop. For example:
.OR.
.AND.
.NOT.
They operate on the logical values true and false as follows:
a .OR. b is true if either a or b (or both) are true otherwise it is false.
a .AND. b is true if both a and b are true otherwise it is false.
Do not assume that a is evaluated first.
.NOT. a is true if a is false and vice-versa.