add test

Author: isaacsas

test/linalg.jl

@@ -18,3 +18,15 @@ N = ModelingToolkit.nullspace(A)
 @test size(N, 2) == 3
 @test rank(N) == 3
 @test iszero(A * N)
+
+A = [0 1 2 0 1 0; 
+     0 0 0 0 0 1;
+     0 0 0 0 0 1;
+     1 0 1 2 0 1;
+     0 0 0 2 1 0]
+col_order = Int[]
+N = ModelingToolkit.nullspace(A; col_order)
+colspan = A[:,col_order[1:4]]   # rank is 4
+@test iszero(ModelingToolkit.nullspace(colspan))
+@test !iszero(ModelingToolkit.nullspace(A[:,col_order[1:5]]))
+@test !iszero(ModelingToolkit.nullspace(A[:,[col_order[1:4]...,col_order[6]]]))