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Why are some signal attributes implicit and others not?

In VHDL, some signal attributes (for example, "TRANSACTION") are implicit signals. Others (like "EVENT"). Why is this?

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The returned VHDL object, its type and value are not limited by language. If user attributes are limited to constant values.

6.7 Attribute declarations An attribute
is a value, function, type, range, signal or constant that may be associated with one or more named objects in the description. There are two categories of attributes: predefined attributes and custom attributes. Predefined attributes provide information about named objects in the description. Clause 16 defines all predefined attributes. Predefined attributes that are signals are not updated.

Custom attributes of an arbitrary type constant . Such attributes are defined by an attribute declaration.

§ 6.7 on p. 92, IEEE Standard VHDL Language Reference, IEEE 1076-2008

, . 'transaction signal bit.

- VHDL. .

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