eloop(expr)
The eloop function is similar to the tloop function. When displaying a GrADS expression (expr) with the E (Ensemble) dimension varying, the
eloop function will evaluate the expr with each designated ensemble member fixed, then reassemble the grids to obtain a final result
that is E-varying. 
The eloop function is provided as a way to obtain E-varying results
from functions or expressions that  are unable to operate
when E is a varying dimension. 
open fcst.ctl
      open  rean.ctl
      set lon -77
      set lat 39
      set e 1 15
      set t 1 last
      d eloop(t2m.1-t2m.2(e=1))