I am having a trouble 1- printing the expression Ad, and 2- using it in a nonlinear constraint.

using LinearAlgebra

using JuMP

model = Model( )n = 8

u = ones(n)@variable(model, A[1:n, 1:n], Symmetric)

A = LowerTriangular(A)

@variable(model, b[1:n])d = @expression(model, A * u)

@expression(model, Ad, (sum(A[i,j]*d[j] for j in 1:i-1) for i in 1:n) )

@NLconstraint(model, sum(b[i]*Ad[i] for i in 1:n) == 1/3);

For the former, when I type Ad in the prompt I get

`Base.Generator{UnitRange{Int64}, var"#37#39"}(var"#37#39"(), 1:8)`

which I don’t know what it means. For the constraint it gives this error

```
MethodError: no method matching getindex(::Base.Generator{UnitRange{Int64}, var"#37#39"}, ::Int64)
Stacktrace:
[1] macro expansion
@ ~/.julia/packages/JuMP/60Bnj/src/macros.jl:1974 [inlined]
[2] top-level scope
@ ./In[4]:15
[3] eval
@ ./boot.jl:373 [inlined]
[4] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
@ Base ./loading.jl:1196
```