pragma Attribute_Definition ([Attribute =>] ATTRIBUTE_DESIGNATOR, [Entity =>] LOCAL_NAME, [Expression =>] EXPRESSION | NAME);
Attribute is a known attribute name, this pragma is equivalent to
the attribute definition clause:
for Entity'Attribute use Expression;
Attribute is not a recognized attribute name, the pragma is
ignored, and a warning is emitted. This allows source
code to be written that takes advantage of some new attribute, while remaining
compilable with earlier compilers.