Specify the abstract data type LIST with the following operations
null:denotes the empty list
add:inserts a sort of NAT into the list
del:deletes an element in the list at a certain position
length:returns the length of the list
append:add two lists together
sum:returns the sum of the natural numbers stored in the list
isEMPTY:return whether the list is empty
specify the NAT and BOOLEAN first
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