Op.RecordsOperations on record types.
type t = Records.ttype atom = VDescr.Descr.Records.Atom.tof_union atoms returns the record component composed of the union atoms.
proj l t returns the (possibly absent) type resulting from the projection on the label l of t.
merge t1 t2 returns the atom resulting from the merging of t1 and t2 (non-absent fields in t2 override those in t1).