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An
expression
consisting of two relations
connected by **and then** or **or else** (a *short-circuit control
form*) shall resolve to be of some boolean type; the
expected type for both relations
is that same boolean type.

The
following logical operators are predefined for every boolean type *T*,
for every modular type *T*, and for every one-dimensional array
type *T* whose component type is a boolean type:

{*AI95-00145-01*}
In many cases, there is a subtype with the correct properties available.
The italicized *T* means:

the first subtype of *T*, for tagged
types;

a subtype of the type *T* without any
constraint or null exclusion, in other cases.

Note that “without a constraint”
is not the same as unconstrained. For instance, a record type with no
discriminant part is considered constrained; no subtype of it has a constraint,
but the subtype is still constrained.

Thus, the last case often is the same as the
first subtype of *T*, but that isn't the case for constrained array
types (where the correct subtype is unconstrained) and for access types
with a null_exclusion
(where the correct subtype does not exclude null).

This italicized *T* is used for defining
operators and attributes of the language. The meaning is intended to
be as described here.

For boolean types, the predefined logical operators
**and**, **or**, and **xor** perform the conventional operations
of conjunction, inclusive disjunction, and exclusive disjunction, respectively.

For modular types, the predefined logical operators
are defined on a bit-by-bit basis, using the binary representation of
the value of the operands to yield a binary representation for the result,
where zero represents False and one represents True. If this result is
outside the base range of the type, a final subtraction by the modulus
is performed to bring the result into the base range of the type.

The logical operators on arrays are performed on
a component-by-component basis on matching components (as for equality
— see 4.5.2), using the predefined
logical operator for the component type. The bounds of the resulting
array are those of the left operand.

The short-circuit control forms
**and then** and **or else** deliver the same result as the corresponding
predefined **and** and **or** operators for boolean types, except
that the left operand is always evaluated first, and the right operand
is not evaluated if the value of the left operand determines the result.

For the logical
operators on arrays, a check is made that for each component of the left
operand there is a matching component of the right operand, and vice
versa. Also, a check is made
that each component of the result belongs to the component subtype. The
exception Constraint_Error is raised if either of the above checks fails.

NOTE The
conventional meaning of the logical operators is given by the following
truth table:

A B (A **and** B) (A **or** B) (A **xor** B)

True True True True False

True False False True True

False True False True True

False False False False False

True True True True False

True False False True True

False True False True True

False False False False False

Next_Car.Owner /= **null** **and** **then** Next_Car.Owner.Age > 25 --* see 3.10.1*

N = 0**or** **else** A(N) = Hit_Value

N = 0

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