TODO

o Functions defined in base R that would need to be explicitly promoted
  to generics in the BiocGenerics package (currently they are implicitly
  made generics by the IRanges package):

  From package base:
    - split(): implicit generic dispatches on (x, f, drop).
      Explicit generic should dispatch on (x, f) only.
    - which(): implicit generic dispatches on (x, arr.ind, useNames).
      Explicit generic should dispatch on (x) only.
    - ifelse(): implicit generic dispatches on (test, yes, no).
      Explicit generic should dispatch on (test) only.
    - nchar(): implicit generic dispatches on (x, type, allowNA).
      Explicit generic should dispatch on (x) only.
    - substr(): implicit generic dispatches on (x, start, stop).
      Explicit generic should dispatch on (x) only.
    - substring(): implicit generic dispatches on (text, first, last).
      Explicit generic should dispatch on (text) only.
    - chartr(): implicit generic dispatches on (old, new, x).
      Explicit generic should dispatch on (x) only.
    - sub(), gsub(): implicit generics dispatch on (pattern, replacement, x,
      ignore.case, perl, fixed, useBytes). Explicit generics should dispatch
      on (x) only.
    - range():
    - by(): 

  From package stats:
    - var(): implicit generic dispatches on (x, y, na.rm, use).
      Explicit generic should dispatch on (x, y) only.
    - cov(): implicit generic dispatches on (x, y, use, method).
      Explicit generic should dispatch on (x, y) only.
    - cor(): implicit generic dispatches on (x, y, use, method).
      Explicit generic should dispatch on (x, y) only.
    - sd(): implicit generic dispatches on (x, na.rm).
      Explicit generic should dispatch on (x) only.
    - median(): implicit generic dispatches on (x, na.rm).
      Explicit generic should dispatch on (x) only.
    - mad(): implicit generic dispatches on (x, center, constant, na.rm,
      low, high). Explicit generic should dispatch on (x) only.
    - IQR(): implicit generic dispatches on (x, na.rm, type).
      Explicit generic should dispatch on (x) only.
    - smoothEnds(): implicit generic dispatches on (y, k).
      Explicit generic should dispatch on (y) only.
    - runmed(): implicit generic dispatches on (x, k, endrule, algorithm,
      print.level). Explicit generic should dispatch on (x) only.