slightly more standard data merge: Symtax.merge (K true) avoids equality on abstract type Pretty.T and gracefully accepts overriding, Symtab.join prefers first entry as usual;
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end