Function is_single_valued 

pub fn is_single_valued(url: &str, resource_type: &str) -> bool

Whether the parameter url produces at most one index value for a resource of type resource_type — as a parameter of that type, or of every type (Resource, DomainResource).

Unknown pairs answer false, which is the safe direction: a caller that treats an unclassified parameter as multi valued is slower, one that treats it as single loses data.