Optimisation deficiency: currval('seq')-->seq scan, constant-->index scan

It seems that optimiser is unaware that currval('seq') can be treated as
a constant within 
an expression and thus produces suboptimal plans for WHERE clauses that
use currval 
thus using a seq scan instead of index scan.

Is it possible (planned) to mark functions as returning a constant when
given a constant 
argument and start using it _as a constant_ (pre-evaluated) in queries

The current behaviour is not very intuitive.

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Hannu