Allow multiple type constraints where permitted by the schema

[james-whiteside]

Problem to Solve

Currently, queries do not permit the following syntax:

$v isa vehicle;
$v isa $vehicle-type;
$v has license-plate "VATICLE";

This syntax allows for the types of the vehicle to be retrieved.

Current Workaround

The following syntax works instead:

$v isa $vehicle-type;
$vehicle-type sub vehicle;
$v has license-plate "VATICLE";

Proposed Solution

The problem with the currently illegal syntax is that $v can have multiple conflicting types, however both types can be satisfied by checking if $vehicle-type is a subtype of vehicle against the schema and returning an error only if not. While not necessary, improving the expressivity of the language in this way (and others) would be a nice feature and might be more intuitive for users.

[james-whiteside]

It's worth noting: this applies to both isa constraints and sub constraints.