anchor.diff:  def can also takes some flags:
    * *-env-params*: pass parameters given to commands in the environement as
                     kak_paramN with N the parameter number
  + * *-shell-params*: pass parameters given to commands as positional parameters
  +                    to any shell expansions used in the command.
    * *-shell-completion*: following string is a shell command which takes
  -                        parameters through the environemnt and output one
  +                        parameters as positional params and output one
                           completion candidate per line.
    * *-allow-override*: allow the new command to replace an exisiting one
                         with the same name.
  
positive.code: 
  Note that these commands are available in interactive command mode, but are
  not that useful in this context.
  
negative.code: 
   * *kak_selection*: content of the last selection
   * *kak_bufname*: name of the current buffer
   * *kak_runtime*: directory containing the kak binary
   * *kak_opt_name*: value of option name
   * *kak_reg_x*: value of register x
  
  