moved structure Keyword to OuterKeyword (Isar/outer_keyword.ML);
support for tagged commands;
tuned theory presentation interface;
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end