US system isolated from SI

doc/DESIGN.md

@@ -150,8 +150,6 @@ struct kilo : units::prefix<kilo, prefix, "k", ratio<1'000>> {};
 struct metre : named_unit<metre, "m", prefix> {};
 struct centimetre : prefixed_unit<centimetre, centi, metre> {};
 struct kilometre : prefixed_unit<kilometre, kilo, metre> {};
-struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio<9'144, 10'000>, metre> {};
-struct mile : named_scaled_unit<mile, "mi", no_prefix, ratio<1'760>, yard> {};
 
 // time
 struct second : named_unit<second, "s", prefix> {};
@@ -160,7 +158,17 @@ struct hour : named_scaled_unit<hour, "h", no_prefix, ratio<3600>, second> {};
 // velocity
 struct metre_per_second : unit<metre_per_second> {};
 struct kilometre_per_hour : deduced_unit<kilometre_per_hour, dim_velocity, kilometre, hour> {};
-struct mile_per_hour : deduced_unit<mile_per_hour, dim_velocity, mile, hour> {};
+
+}
+
+namespace units::us {
+
+// length
+struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio<9'144, 10'000>, si::metre> {};
+struct mile : named_scaled_unit<mile, "mi", no_prefix, ratio<1'760>, yard> {};
+
+// velocity
+struct mile_per_hour : deduced_unit<mile_per_hour, si::dim_velocity, mile, si::hour> {};
 
 }
 ```