Composing quantities for output

[kwikius]

Ultimately stream output is not the concern of the library in general. It is up to the programmer, but for use in examples etc, there needs to be an output scheme, and that can be used to provide some ideas for users own output schemes

For SI output there is some loose guidance
https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-6-rules-and-style-conventions-printing-and-using
so there probably needs to be a default output that conforms to those guidelines

6.1.5 Unit symbols obtained by multiplication
6.1.6 Unit symbols obtained by division

Activities associated with composing SI stream output.
naming
multiplication
division
pow

simple grammar

expression :
prim |
multiplicative_expression

prim:
named_quantity |
( expression ) |
pow<N,D>( expression )

multiplicative_expression :

prim * multiplicative_expression |
prim / multiplicative_expression

Composition is done at the level of units, not quantities, otherwise quantities template size will increase in complexity according to the expression in source code

[kwikius]

Let us aim to do the minimum of composition with operators or provide it as an example of how units could be csutomised. It greatly increases the size of the quantity signature for error messages.[1] It is cool over simplicity. Can just as easily do a name, not as cool, but just as effective.

[1] Actually this is hidden by derivation so may not be that much of an issue after all....

[kwikius]

named dimension can be used to provide an unprefixed unit symbol in a particular measurement system
similar to the base_unit symbol mechanism

// named dimension
      struct abstract_force_t : decltype ( abstract_mass_v * abstract_acceleration_v){};
      
       template <typename CharSet>
       get_named_dimension_symbol<abstract_force_t,si_measurement_system,CharSet> = "N";

This enables the symbol to be customised/extended to future encodings and languages without modifying multiple units directly. Defintely looks like a good idea. Also add prefixing functionality in same way