misc tuning and simplification, using Isabelle_Sidekick.Asset (not sidekick.Asset, which is dynamically dispatched to slightly different semantics);
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end