ensure addedoperator doesn't nest

Author: vpuri3

src/basic.jl

@@ -330,8 +330,9 @@ AddedOperator(L::AbstractSciMLOperator) = L
 # constructors
 Base.:+(A::AbstractSciMLOperator, B::AbstractMatrix) = A + MatrixOperator(B)
 Base.:+(A::AbstractMatrix, B::AbstractSciMLOperator) = MatrixOperator(A) + B
-Base.:+(ops::AbstractSciMLOperator...) = AddedOperator(ops...)
 
+Base.:+(ops::AbstractSciMLOperator...) = reduce(+, ops)
+Base.:+(A::AbstractSciMLOperator, B::AbstractSciMLOperator) = AddedOperator(A, B)
 Base.:+(A::AbstractSciMLOperator, B::AddedOperator) = AddedOperator(A, B.ops...)
 Base.:+(A::AddedOperator, B::AbstractSciMLOperator) = AddedOperator(A.ops..., B)
 Base.:+(A::AddedOperator, B::AddedOperator) = AddedOperator(A.ops..., B.ops...)

test/basic.jl

@@ -171,6 +171,15 @@ end
 
     v=rand(N,K); @test mul!(v, op, u) ≈ (A+B) * u
     v=rand(N,K); w=copy(v); @test mul!(v, op, u, α, β) ≈ α*(A+B)*u + β*w
+
+    # ensure AddedOperator doesn't nest
+    A = MatrixOperator(rand(N, N))
+    L = A + (A + A) + A
+    @test L isa AddedOperator
+    for op in L.ops
+        @test !isa(op, AddedOperator)
+    end
+
 end
 
 @testset "ComposedOperator" begin
@@ -221,7 +230,7 @@ end
     v=rand(N,K); @test ldiv!(v, op, u) ≈ (A * B * C) \ u
     v=copy(u);   @test ldiv!(op, u)    ≈ (A * B * C) \ v
 
-    # ensure composedoperators don't nest
+    # ensure composedoperators doesn't nest
     A = MatrixOperator(rand(N, N))
     L = A * (A * A) * A
     @test L isa ComposedOperator