Use of Σ is not consistent

[odow]

Someone complained (rightly) about:

julia> model = Model();

julia> @variable(model, x[1:2])
2-element Vector{VariableRef}:
 x[1]
 x[2]

julia> @expression(model, Σ(x[i] for i in 1:2))
x[1] + x[2]

julia> @expression(model, Σ(x))
ERROR: UndefVarError: `Σ` not defined
Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/MutableArithmetics/cnvbo/src/rewrite.jl:321 [inlined]
 [2] macro expansion
   @ ~/.julia/dev/JuMP/src/macros.jl:717 [inlined]
 [3] top-level scope
   @ REPL[5]:1

cc @dourouc05

[odow]

@dourouc05 you should tell the person not to use the un-documented Σ feature 😄