xo-unit: minor doc improvements

docs/implementation.rst

@@ -30,7 +30,10 @@ Abstraction tower for *xo-unit* components.
 - scaled_unit:
   a unit involving zero or more dimensions, and associated conversion factor.
 
-  - expresses result of arithmetic involving multiple scales.
+  - can express result of arithmetic involving multiple scales,
+    by reporting an outer scalefactor
+  - a scaled unit is 'natural' if its outer scalefactor is 1.
+  - quantities are represented by associating a natural scaled_unit instance
   - scaled_units are closed under multiplication and division.
   - multiplication and division commit to a single :code:`basis_unit` for each
     dimension.

utest/quantity.test.cpp

@@ -483,6 +483,7 @@ namespace xo {
             constexpr auto q2 = detail::quantity_util::divide(ms, ms);
 
             /* proof that q2 is constexpr */
+            static_assert(q2.is_dimensionless());
             static_assert(q2.scale() == 1.0);
             static_assert(q2.unit().n_bpu() == 0);
             static_assert(q2.unit()[0].power() == power_ratio_type(0,1));
@@ -490,6 +491,7 @@ namespace xo {
             constexpr auto q3 = ms / ms;
 
             /* proof that q3 is constexpr */
+            static_assert(q3.is_dimensionless());
             static_assert(q3.scale() == 1.0);
             static_assert(q3.unit().n_bpu() == 0);
             static_assert(q3.unit()[0].power() == power_ratio_type(0,1));