12 Relvar versus attribute predicates

In the Relational Model (RM), each relvar has an associated predicate. For the supplier-and-parts database, there is a relvar recording suppliers: 

VAR S BASE RELATION
{
    S#     CHAR,
    SNAME  CHAR,
    STATUS INTEGER,
    CITY   CHAR
}
KEY { S# };

and its predicate is:

S(S#,SNAME,STATUS,CITY)
⇔
There exists a supplier identified by supplier number S# with name SNAME having status STATUS and located in city CITY

In Predica this relvar is represented by entity Supplier together with a reference from entity ShipmentsSystem: 


entity Supplier
{
    String id;
    String name;
    Integer status;
    String city;
    key {id};
}

entity ShipmentsSystem
{
    Supplier supplier*;
    ...
}

The Supplier attributes are predicates, simpler and more atomic than the relvar predicate of the RM:

id(I)
⇔
The supplier has id I
name(N)
⇔
The supplier has name N
status(S)
⇔
The supplier has status S
city(C)
⇔
The supplier is located in city C

and in the ShipmentsSystem there is a predicate for existence of a supplier:

supplier(S)
⇔
The shipments system has a supplier S