Messed up size accounting panic

[yav]

The following expression causes a panic:

[ x+y | x <- [(0:Integer)...], y <- [1] ]

 Location:  lookupSeqMap
  Message:   Messed up size accounting
             Nat 1
             1

[yav]

After merging PR #1557, this does not panic, but there is still something wrong with the list comprehension. In particular,
indexing in it returns the correct result (e.g., test @ i works fine for any i), but trying to just print the expression, or take more than one element complainsabout invalid sequence index: 1