A trust created by will may be revoked at the pleasure of the testator. A trust otherwise created can be revoked only--
     (a) where all the beneficiaries are competent to contract--by their consent;
     (b) where the trust has been declared by a non-testamentary instrument or by word of mouth--in exercise of a power of revocation expressly reserved to the author of the trust; or
     (c) where the trust is for the payment of the debts of the author of the trust, and has not been communicated to the creditors--at the pleasure of the author of the trust.
     Illustration
     A conveys property to B in trust to sell the same and pay out of the proceeds the claims of A's creditors. A reserves no power of revocation. If no communication has been made to the creditors, A may revoke the trust. But if the creditors are parties to the arrangement, the trust cannot be revoked without their consent.