Application-time periods: foreign key

The syntax as follows:
{code}<referential constraint definition> ::= FOREIGN KEY
<left paren> <referencing column list>
[ <comma> <referencing period specification> ]
<right paren> <references specification>
<references specification> ::= REFERENCES <referenced table and columns>...

<referencing period specification> ::= PERIOD <application time period name>
<referenced table and columns> ::= <table name>
[ <left paren> <referenced column list>
[ <comma> <referenced period specification> ]
<right paren> ]
<referenced period specification> ::= PERIOD <application time period name>
{code}
 
-This task looks very much like a semantic one, i.e. it should be possible to create such a reference to period-driven primary keys. This syntax adds no complex logic, like the PRIMARY KEY does.