fix @d for only .=

[tiemvanderdeure]

This fixes @d da1 .= da2, which would error because arg was used but not defined.

Hope this makes sense, I really have no clue what I'm doing here :)

[rafaqz]

Ah must be from my recent "fix". Looks right to me

dump(expr) and @macroexpand @d are good tools here to get your head around things. AST is pretty simple but there's a lot of nesting to wade through.