Sometimes a TransferPart in an atomicRearrange only expresses what amounts
should be given to a seat, leaving it to other TransferPart of the
same atomicRearrange to balance it out. For this case, the
[undefined, toSeat, undefined, toAmounts] form is more clearly expressed as
toOnly(toSeat, toAmounts). Unlike TransferPart, both arguments to
toOnly are non-optional, as otherwise it doesn't make much sense.
Sometimes a TransferPart in an atomicRearrange only expresses what amounts should be given to a seat, leaving it to other TransferPart of the same atomicRearrange to balance it out. For this case, the
[undefined, toSeat, undefined, toAmounts]form is more clearly expressed astoOnly(toSeat, toAmounts). Unlike TransferPart, both arguments totoOnlyare non-optional, as otherwise it doesn't make much sense.