><span class="identifier">Y</span></code>), with the
      latter defaulting to double-precision floating point if not otherwise specified.
      The value type must have a normal copy constructor and copy assignment operator.
      Operators +, -, *, and / are provided for algebraic operations between scalars
      and units, scalars and quantities, units and quantities, and between quantities.
      In addition, integral and rational powers and roots can be computed using the
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