Handle nested diff

Author: YingboMa

src/utils.jl

@@ -193,8 +193,17 @@ makesym(t::Num; kwargs...) = makesym(value(t); kwargs...)
 
 function lower_varname(var::Term, idv, order)
     order == 0 && return var
-    name = Symbol(nameof(var.op), :ˍ, string(idv)^order)
-    return Sym{symtype(var.op)}(name)(var.args[1])
+    name = string(nameof(var.op))
+    underscore = 'ˍ'
+    idx = findlast(underscore, name)
+    append = string(idv)^order
+    if idx === nothing
+        newname = Symbol(name, underscore, append)
+    else
+        nidx = nextind(name, idx)
+        newname = Symbol(name[1:idx], name[nidx:end], append)
+    end
+    return Sym{symtype(var.op)}(newname)(var.args[1])
 end
 
 function lower_varname(t::Term, iv)

test/derivatives.jl

@@ -4,10 +4,11 @@ using Test
 # Derivatives
 @parameters t σ ρ β
 @variables x y z
-@variables uu(t)
+@variables uu(t) uuˍt(t)
 @derivatives D'~t D2''~t Dx'~x
 
 @test Symbol(D(D(uu))) === Symbol("uuˍtt⦗t⦘")
+@test Symbol(D(uuˍt)) === Symbol(D(D(uu)))
 
 test_equal(a, b) = @test isequal(simplify(a), simplify(b))