how to write a definition of variable

@Huy , as your condition is dependent on the index domain and not on a variable, you could also use an if block in the definition of variable x(t). 

if ord(t) = 1 then

0.3

else

x(t-1) + y(t)

endif

This will generate the below constraints - assuming that the elements in set T are {0,1,2,3,...}

x(0) = 0.3

x(1) = x(0) + y(1)

x(2) = x(1) + y(2)

….