This is the default value to use when the key in $options is empty.
If not set the option key is required! If the default value is a Closure the closure is called
and it's result is used as value.
The callback receives $key, $options, $definition, $path(For child arrays)
This is the default value to use when the key in $options is empty. If not set the option key is required! If the default value is a Closure the closure is called and it's result is used as value. The callback receives $key, $options, $definition, $path(For child arrays)