Function instantiate 

pub fn instantiate(
    scheme: &SchemeSpec,
    bindings: &HashMap<String, String>,
) -> Result<SchemeInstance, Error>

Instantiate a scheme with concrete bindings.

Every premise slot in the scheme must have a corresponding entry in bindings. Returns Error::MissingBinding if any required slot is unbound.

Also available as SchemeSpec::instantiate (which delegates here).