code: public static final Property propertyFor(RelationshipType rtype) {
          if (rtype == null) {
              throw new InvalidArgument("rtype", rtype);
          }
  
          return TYPETOPROPERTY.get(rtype);
      }
docs: Retrieve the RDF {@link Property} for the BEL {@link RelationshipType}.
  
  @param rtype {@link RelationshipType}, the relationship type, which
  cannot be null
  @return {@link Property} the property for the {@link RelationshipType}
  @throws InvalidArgument Thrown if <tt>rtype</tt> is null