Function key_prefix 

pub fn key_prefix(
    order: IndexOrder,
    e: Option<EntityId>,
    a: Option<AttrId>,
    v: Option<&Value>,
) -> Vec<u8> 

Builds the encoded key prefix for a partial datom in one index order.

Components are consumed in the index’s component order and encoding stops at the first missing component, so the result is a proper range prefix.