4. Elements of QUEL Statements : Qualifications : Logical Operators
 
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Logical Operators
The following Boolean logical operators are recognized in QUEL:
not
(negation)
and
(conjunction)
or
(disjunction)
These operators take and return truth functions (true, false or unknown).
Not has the highest precedence of the three operators; and and or have equal precedence. Parentheses can be used to override the default order of processing; by default logical operators are processed from left to right.