tif returns the tif code for x, while tifName
  returns a name for that code.  Many of the codes have several names,
  but only the default one is returned.
tif or tifName called with no arguments returns a vector
  of all tif codes with names.
period returns a vector like z giving the number of
  periods elapsed since the first period defined for its argument's
  frequency.
basePeriod returns the ti for the first period defined
  for tif(x).