obj
IEquatable<preterm>
IStructuralEquatable
IComparable<preterm>
IComparable
IStructuralComparable
Ptmabs preterm * preterm
preterm * preterm
Ptmcomb preterm * preterm
Ptmconst string * pretype
string * pretype
Ptmtyped preterm * pretype
preterm * pretype
Ptmvar string * pretype