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Explicitly track precision of unit conversion factory
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Some conversion factors between units (particularly historical units) are approximate. The unit conversion facility should know about such uncertainties, and take them into account when converting between units (just like it takes into account the uncertainty of the values that get converted). Doing so will allow us to support (approximate) conversion even for historical units that have no exact conversion to SI units (e.g. x).

Implementation note: On Items that describe units of measurement, the conversion factor is given by Statements about property P2370. The Quantity value associated with the Statement can already represent an uncertainty. The unit conversion facilities should make use of this information.