unique value - trigger?

Hi folks,

I'm back with my lnumbers table again.

nymr=# \d lnumbers              Table "lnumbers" Column   |         Type          | Modifiers
-----------+-----------------------+-----------lnid      | integer               | not nulllnumber   | character
varying(10)| not nulllncurrent | boolean               |
 
Primary key: lnumbers_pkey
Triggers: RI_ConstraintTrigger_7575462

While each loco can have a number of different numbers, only one can be 
current at any one time.

I want to make it so that if I set lncurrent to true for one row, any existing 
true rows are set to false.

I'm guessing that I need to create a trigger to be actioned after an insert or 
update which would update set lncurrent=false where lnid not = <current lnid> 
but I can't seem to sus it put.

-- 
Gary Stainburn
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