[FileFormats.NL] support common subexpressions

[GiovanniKarra]

On this line, if common subexpressions are detected, an error is thrown

This makes it impossible to load the whole CUTE collection from NL files into JuMP models.

For example, the scurly10.mod problem has these variables

# scurly10.mod
...
var x{i in 1..N} := 0.0001*scale[i]/(N+1);
var y{i in 1..N-K} = sum {j in i..i+K} x[j]*scale[j];
var z{i in N-K+1..N} = sum {j in i..N} x[j]*scale[j];
...

which seem to trigger the error.

My use case is a library that allows loading the CUTE problems into JuMP models, called CUTE.jl, and in order to do so I convert all the AMPL files into NL files and load them using the JuMP.read_from_file function.

[odow]

Yeah I've been waiting for someone to request this 😄