## How do I use IterativeSolvers solvers with a weighted tolerance vector?

IterativeSolvers.jl computes the norm after the application of the left precondtioner Pl. Thus in order to use a vector tolerance weights, one can mathematically hack the system via the following formulation:

using LinearSolve, LinearAlgebra
Pl = LinearSolve.InvPreconditioner(Diagonal(weights))
Pr = Diagonal(weights)

A = rand(n,n)
b = rand(n)

prob = LinearProblem(A,b)
sol = solve(prob,IterativeSolvers_GMRES(),Pl=Pl,Pr=Pr)

If you want to use a "real" preconditioner under the norm weights, then one can use ComposePreconditioner to apply the preconditioner after the application of the weights like as follows:

using LinearSolve, LinearAlgebra
Pl = ComposePreconitioner(LinearSolve.InvPreconditioner(Diagonal(weights),realprec))
Pr = Diagonal(weights)

A = rand(n,n)
b = rand(n)

prob = LinearProblem(A,b)
sol = solve(prob,IterativeSolvers_GMRES(),Pl=Pl,Pr=Pr)