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Sequences
🌌Sequences
Sequences -- unit type defining features related to Sequence but not requiring
an instance
monoid of Sequences with infix concatentation operation.
create an empty list
get the index of x within this list or nil if it does not exist
determine the index of element x within list l. 0 if x is at the head
of the list, 1 if it comes directly after head, etc. nil if x is not
in the list.