做DDD，如下代码，报错maxvar too small
reghdfe lev Treat_y2003_highind  Treat_y2004_highind Treat_y2006_highind Treat_y2007_highind Treat_y2008_highind i.ind_year i.prov_year i.ind_prov, cluster(coid) absorb(coid)

报错：
maxvar too small
    You have attempted to use an interaction with too many levels or attempted to fit a model with too many variables.  You need to
    increase maxvar; it is currently 5000.  Use set maxvar; see help maxvar.

    If you are using factor variables and included an interaction that has lots of missing cells, try set emptycells drop to reduce the
    required matrix size; see help set emptycells.

    If you are using factor variables, you might have accidentally treated a continuous variable as a categorical, resulting in lots of
    categories.  Use the c. operator on such variables.
r(907);