renamed parents_of to parents_of_name to avoid name clash with function
from thm.ML
Pcpo = Porder +
classes pcpo < po
arities void :: pcpo
consts
UU :: "'a::pcpo"
rules
minimal "UU << x"
cpo "is_chain(S) ==> ? x. range(S) <<| (x::'a::pcpo)"
inst_void_pcpo "(UU::void) = UU_void"
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end