Data::Sah::DefaultValue generates code to set default value using value rules.

A value rule is put in Data::Sah::Value::$COMPILER::$TOPIC::$DESCRIPTION module,
for example: Data::Sah::Value::perl::Perl::this_mod contains the value "this
module" (see App::ThisDist for more details on the meaning of "this module").

Basically, a value rule will provide an expression (expr_value) that return some
value.