Incorrect results with lead/lag over windows with duplicates

LEAD/LAG should be computed as row values functions, rather than over a range.

for example:

select e1, lead(e1, 2) over (order by e1 nulls last) from pm1.g1

returns three rows of ('a', 'c') - rather than the expected ('a', 'a'), ('a', 'b'), ('a', 'c')