LPO + MPO => RPO

[jwaldmann]

we have LPO (compare arguments lexicographically). we should also allow
"multiset status" (compare arguments as multiset). This results in what people usually call RPO. Technically,

Precedence = Map symbol Height

will be changed to

Precedence = Map symbol (Status,Height)
data Status = Lex | Multi

cf. http://verify.rwth-aachen.de/giesl/papers/FROCOS07-distribute.ps
but they put the permutation into Lex while we do it in the argument filter
(and want to keep it there).