Macro define_global_property 

macro_rules! define_global_property {
    ($global_property:ident, $value:ty, $validate: expr) => { ... };
    ($global_property: ident, $value: ty) => { ... };
}

Defines a global property with the following parameters:

• $global_property: Name for the identifier type of the global property
• $value: The type of the property’s value
• $validate: A function (or closure) that checks the validity of the property and returns Result<(), Box<dyn std::error::Error + 'static>> (optional)

Validator code is client code, so it should create and box its own error values. Ixa wraps any returned error in IxaError::IllegalGlobalPropertyValue when the property is set or loaded.