refrain from SIGHUP handling (cf. 5f629ee2502b), which does not work on Cygwin and appears to be redundant anyway (no extra output produced within pipe);
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end