Add helper function to check whether a function arg is 'X' or '_X'
Add helper function to check whether a function arg is 'X' or '_X'

Also allow '_'.

rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix