Sum across dimensions using the following format:  

sum(VariableName[set] for set in Sets)


Example:

years = collect(Future:Final)

for area in Areas, ec in ECs, enduse in Enduses, year in years

  xMMSFTotal[enduse,ec,area,year] = 

    sum(xMMSF[enduse,tech,ec,area,year] for tech in Techs)

end