references definition to multi-field primary key

Hi all,

I've got some fuel pumps, each of which sell a number of fuel grades - one 
nozzle per grade, each nozzle connected to a specified tank containing that 
grade.

I can define the tanks, the pump numbers, and the pump grade/nozzle config 
using the tables below.

create table grades ( -- different types of fuel sold gid character, gdesc varchar(20), gprice float, primary key
(gid)
);

create table tanks (  -- storage tanks tid int4 not null, tgrade character references grades(gid), primary key (tid)
);

create table pumps ( -- list of pumps pid int4 not null, primary key (pid)
);

create table pgrades ( -- list of nozzles/grades per pump pgpid int4 not null references pumps(pid),   pgnozzle int4
notnull, pgtank int4 not null references tanks(tid), primary key (pgpid, pgseq)
 
);

My problem is that I want to be able to define a 'Pump Readings' table to show 
per pump/nozzle the opening and closing reading.  However, my problem is that 
I don't know how to define the references so that I can only create a 
preadings row for an existing pgrages entry.  Here's the table less the 
required references entry.

create table preadings ( -- daily reading per pump/nozzle  prdate date not null,  prpump int4 not null  prnozzle int4,
propen integer,  prclose integer,  primary key (prdate, prpump, prseq)
 
);

I only want the insert to work if prpid matches pgpid and prnozzle matches 
pgnozzle.

-- 
Gary Stainburn
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