Print detected transformation equations when there is not exactly one

Author: hersle

src/systems/diffeqs/basic_transformations.jl

@@ -137,7 +137,7 @@ function change_independent_variable(sys::AbstractODESystem, iv, eqs = []; dummi
     eqs = [eqs; get_eqs(sys)] # all equations (system-defined + user-provided) we may use
     idxs = findall(eq -> isequal(eq.lhs, D1(iv2_of_iv1)), eqs)
     if length(idxs) != 1
-        error("Exactly one equation for $D1($iv2_of_iv1) was not specified!")
+        error("Exactly one equation for $D1($iv2_of_iv1) was not specified! Got $(length(idxs)) equations:\n", join(eqs[idxs], '\n'))
     end
     div2_of_iv1_eq = popat!(eqs, only(idxs)) # get and remove e.g. du/dt = ... (may be added back later as a dummy)
     div2_of_iv1 = chain_rule(div2_of_iv1_eq.rhs)

test/basic_transformations.jl

@@ -148,8 +148,10 @@ end
     M = complete(M)
     @test_throws "singular" change_independent_variable(M, M.x)
     @test_throws "structurally simplified" change_independent_variable(structural_simplify(M), y)
-    @test_throws "not specified" change_independent_variable(M, w)
-    @test_throws "not specified" change_independent_variable(M, v)
+    @test_throws "Got 0 equations:" change_independent_variable(M, w)
+    @test_throws "Got 0 equations:" change_independent_variable(M, v)
+    M = ODESystem([D(x) ~ 1, v ~ 1], t; name = :M) |> complete
+    @test_throws "Got 2 equations:" change_independent_variable(M, M.x, [D(x) ~ 2])
     @test_throws "not a function of the independent variable" change_independent_variable(M, y)
     @test_throws "not a function of the independent variable" change_independent_variable(M, z)
     M = ODESystem([D(x) ~ 0, v ~ x], t; name = :M)