Sec. 671. Trust income, deductions, and credits attributable to grantors and others as substantial owners. 672. Definitions and rules. 673. Reversionary interests. 674. Power to control beneficial enjoyment. 675. Administrative powers. 676. Power to revoke. 677. Income for benefit of grantor. 678. Person other than grantor treated as substantial owner. 679. Foreign trusts having one or more United States beneficiaries.