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