fold type issues

[thisbefruit]

melorun> for x in [[1], [2], [3]] fold accum = [] with accum ++ x
Melodeon compilation failed
/home/thisbefruit/Themelio/.melorun.melo.tmp:3: error: post-iteration type [{1}] | [{2}, {1}] | [{3}, {1}] | [{2}, {1}] fundamentally incompatible wth pre-iteration type []
	for x in [[1], [2], [3]] fold accum = [] with accum ++ x
	^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

melorun> for x in [[1],[2],[3]] fold accum=[] :: [Nat;0] with accum ++ x
Melodeon compilation failed
/home/thisbefruit/Themelio/.melorun.melo.tmp:3: error: post-iteration type [Nat; (@g20 + 1)] | [{2}, {1}] | [{3}, {1}] | [{2}, {1}] fundamentally incompatible wth pre-iteration type [Nat; @g20]
	for x in [[1],[2],[3]] fold accum=[] :: [Nat;0] with accum ++ x
	^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~