What is the test for constructive trust in the context of a mutual will?

58. It is also my respectful view that the remedy of constructive trust founded on the principle of unjust enrichment is not analogous to the principles enunciated in Dufour v. Pereira, supra, and that the trial judge erred when he concluded that an unjust enrichment was required. A constructive trust arising from an unjust enrichment is imposed on property gained at the expense of another for no juristic reason, whereas the obligation created by an agreement not to revoke mutual wills binds not only that portion of the survivor's estate which may have come from the estate of the first to die, but also his or her own property.