Lagged values
l.var = l1.var = variable at t-1
l2.var = variable at t-2
l(1/3).var = l1.var l2.var l3.var
etc.
Leading values
f.var = f1.var = variable at t+1
f2.var = variable at t+2
Differences
d.var = var[t] - var[t-1]
d2.var = ( var[t] - var[t-1] ) - (var[t-1] - var[t-2])
Seasonal differences
s.var = var[t] - var[t-1]
s4.var = var[t] - var[t-4]
