D.C. Code § 42-513
Two or more future estates may be created to take effect in the alternative, so that if the first in order shall fail to vest the next in succession may be substituted for it and take effect accordingly.
Mar. 3, 1901, 31 Stat. 1352, ch. 854, § 1026
1973 Ed., § 45-813.
1981 Ed., § 45-213.