USAGE:
mgarch(formula.mean=~arma(0,0), formula.var=~garch(0,0),
series=NULL, series.start=1, x=NULL, x.start=1,
xlag=0, z=NULL, z.start=1, model=NULL, leverage=F,.....)
OPTIONAL ARGUMENTS:
z
a vector or matrix of exogenous variables in the variance equation.
Preferably this should be specified in formula.var