This is not implemented on Sequence because, at least for lists and
strings, I think users expect an eager result. Multiplying a string
should give you back a string, not a lazy sequence of repeated
characters.
This also mirrors "+" on strings and lists, which is eager. I like the
idea of having a general guideline that operators are eager.
Repetition is useful for arbitrary sequences, but for that maybe we
should add a "repeat()" method.
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