sequentially adjacent to is downstream of sequence of is immediately downstream of sequence of is immediately downstream of sequence of x is immediately downstream of the sequence of y if and only if either (1) x and y have sequence units, and all units of x are downstream of all units of y, and x is sequentially adjacent to y, or (2) x and y are sequence units, in which case the immediately downstream relation is primitive and defined by context: for DNA bases, y would be adjacent and 5' to y [RO] is immediately upstream sequence of