Given an iterable of offset vectors, return the shortest list of the
unique n-instrument offset vectors from which all the vectors in the
input iterable can be constructed. This can be used to determine the
minimal set of n-instrument coincs required to construct all of the
coincs for all of the requested instrument and offset combinations in a
set of offset vectors.
It is assumed that the coincs for the vector {"H1": 0,
"H2": 10, "L1": 20} can be constructed from the
coincs for the vectors {"H1": 0, "H2": 10} and
{"H2": 0, "L1": 10}, that is only the relative
offsets are significant in determining if two events are coincident, not
the absolute offsets.
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