Document: You don't (Have to re-declare conditions on each repetition)

In my derive_builder efforts, I have hit this unpleasantness:

struct FooBuilder {
  $(
     ${define ... /* various complex conditions */ }
     
     $fname: Option<$ftype>,
  )
}

impl FooBuilder {
   $(
     ${define ... /* those same complex conditions, copied again! */ }

     pub fn $fname(&mut self, value: $ftype) -> &mut Self { ... }
   )

   pub fn build(&self) -> Result<Self, FooBuilderError> {
     Ok(Self {
        $(
            ${define ... /* those same complex conditions, copied a third time! */ }    
            $fname: self.$fname.clone().ok_or(...)?,
        )
     })
   }
}

As you can see, it would be convenient if there were some way to extract those definitions into some common location. But right now, they are only scoped within a single repetition. I don't believe I can lift time to the top level, since they refer to field-specific things like fmeta.

I wonder if this is something that #36 might be able to solve...?