named parameters in SQL functions

At Tom's suggestion I am looking at allowing use of parameter names in 
SQL functions instead of requiring use of $1 etc. That raises the 
question of how we would disambiguate a parameter name from a column 
name. Essentially, ISTM, we could use some special marker such as @ 
(c.f. SQL Server) or : (c.f. ecpg) or else we could have some rule that 
says which name takes precedence. I think I prefer a special marker, 
other things being equal. Is there a standard on this?

cheers

andrew