+## WARNING_HEADER(tag, msg)
+##
+## Add a warning header to the current mesage.
+m4_define(<:WARNING_HEADER:>,
+<:add_header = X-CONF_header_token-Warning: $1 $2:>)
+
+## _LOOKUP_ARGS([if-found], [if-not-found], [default])
+##
+## Processing for lookup arguments. Use as
+##
+## ${lookup {K} ST {F} _LOOKUP_ARGS(...)}$.
+##
+## IF-FOUND defaults to `{$value}'; IF-NOT-FOUND defaults to DEFAULT, which
+## defaults to `{}'.
+m4_define(<:_LOOKUP_ARGS:>,
+ <:m4_ifelse(<:$1$2$3:>, <::>, <::>,
+ <:$1$2:>, <::>, <:{$value}$3:>,
+ <:$2:>, <::>, <:$1$3:>,
+ <:$1$2:>):>)
+