6.7 Null Procedures
The execution of a null procedure is invoked by a
subprogram call. For the execution of a subprogram call on a null procedure,
the execution of the subprogram_body
has no effect.
The elaboration of a null_procedure_declaration
has no other effect than to establish that the null procedure can be
called without failing the Elaboration_Check.
Simplify(Expr : in out
Expression) is null
; -- see 3.9
-- By default, Simplify does nothing, but it may be overridden in extensions of Expression
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