lingo中如何输入求和

sets:
S/1..11/:x,L,c;
T/1..17/:a;
endsets
data:
L=160.0,153.4,140.9,132.0,122.7,112.5,102.7,91.6,82.7,74.5,73.7;
c=9.0,9.1,9.2,9.3,9.4,9.5,9.6,9.7,9.8,9.9,10;
a=1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17;
enddata
怎么输入下面的式子？
min z=L-∑c(i)*x(i)
L/10<=∑x(i)∑c(i)*x(i)<=L
X6>=a

求和的符号用sum代替