u_str macro

[bc0n]

How does this work in Unitful and DynamicQuantities?

• I need to be able to look up a unit type from the unit label, but since these are not 'registered' with the package do I have to search the type tree?
  No, if I make the the unit labels into functions, I can just blindly call the label-function: 3u"mm" -> "mm" -> mm(1) -> 3*mm(1).
  Since */-^ are not valid function identifiers, I can't make literal label-functions for complex units like Kg-m/s^2. Instead I need to insert logic into u_str to break that into atomic label functions and return Kg(1)*m(1)/s(1)^2.

• How does u_str() work?
  With a little magic; simply defining a macro u_str(arg) leads to the correct handling of 3u"mm", there must be some internal stripping of _str and parsing of 3u"mm" into 3 * (return of macro u_str("mm")).