todo

ConfigureReactive

if you add a source in the middle of a struct, should it be able to fill
fields later in the struct?

"Magic" values that rewrite.

Handle INI files: https://pkg.go.dev/github.com/go-ini/ini#pkg-overview

nflex: .ini & .xml -- a wrapper for general unmarshal

global registry

validate tests

validation of flags causes usage error

issue with file filler should be blamed on the config file

split deindent out to its own package

test double-level nested structs

rewrites and loops as layer over nflex

combining slices between file filler, flag, and env
combining maps between file filler and flag

support Len for flags
support Keys for flags (also needs Recurse?)

support struct filling for flags with prefix as Recurse

support "content=application/json" for flags/environment variables


support CUE (cuelang.org); Jsonnet; GCL; HCL; and TOML.