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Additional material

size(nil) ->> 0.
size(push(X, S)) ->> size(S) + 1.
size(error) ->> error.

empty(nil) ->> true.
empty(push(X, S)) ->> false.
empty(error) ->> error.

top(nil) ->> error.
top(push(X, S)) ->> X.
top(error) ->> error.

pop(nil) ->> error.
pop(push(X, S)) ->> S.
pop(error) ->> error.

size(nil) ->> 0.
size(add(X, Q)) ->> size(Q) + 1.
size(error) ->> error.

empty(nil) ->> true.
empty(add(X, Q)) ->> false.
empty(error) ->> error.

first(nil) ->> error.
first(add(X, Q)) ->> if(Q = nil, X, first(Q)).
first(error) ->> error.

drop(nil) ->> error.
drop(add(X, Q)) ->> if(Q = nil, nil, add(X, drop(Q))).
drop(error) ->> error.

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