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).