-theorem fold_sum: ∀A,B. ∀I,J:list A.∀nil.∀op:Aop B nil.∀f.
- op (\fold[op,nil]_{i∈I} (f i)) (\fold[op,nil]_{i∈J} (f i)) =
- \fold[op,nil]_{i∈(I@J)} (f i).
+theorem fold_sum: ∀A,B. ∀I,J:\ 5a href="cic:/matita/basics/list/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A.∀nil.∀op:\ 5a href="cic:/matita/basics/list/Aop.ind(1,0,2)"\ 6Aop\ 5/a\ 6 B nil.∀f.
+ op (\ 5a title="\fold" href="cic:/fakeuri.def(1)"\ 6\fold\ 5/a\ 6[op,nil]_{i∈I} (f i)) (\ 5a title="\fold" href="cic:/fakeuri.def(1)"\ 6\fold\ 5/a\ 6[op,nil]_{i∈J} (f i)) \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6
+ \ 5a title="\fold" href="cic:/fakeuri.def(1)"\ 6\fold\ 5/a\ 6[op,nil]_{i∈(I\ 5a title="append" href="cic:/fakeuri.def(1)"\ 6@\ 5/a\ 6J)} (f i).