I am manipulating partial differential equations symbolically, and would like to get the easily readable form $\rho \frac{\partial v}{\partial t}$, leaving variables implicit.

Based on suggestions from [How to make traditional output for derivatives](https://mathematica.stackexchange.com/questions/23659/how-to-make-traditional-output-for-derivatives) I started out with 

```
Derivative /: 
  MakeBoxes[Derivative[α__][f1_][vars__Symbol], TraditionalForm] := 
    Module[{bb, dd, sp},
      MakeBoxes[dd, _] ^= 
      If[Length[{α}] == 1, "\[DifferentialD]", "∂"];
      MakeBoxes[sp, _] ^= "\[ThinSpace]";
      bb /: MakeBoxes[bb[x__], _] := RowBox[Map[ToBoxes[#] &, {x}]];
      TemplateBox[{ToBoxes[bb[dd^Plus[α], f1]], 
      ToBoxes[Apply[bb, 
        Riffle[Map[bb[dd, #] &, 
        Select[({vars}^{α}), (# =!= 1 &)]], sp]]], 
        ToBoxes[Derivative[α][f1][vars]]}, "ShortFraction", 
        DisplayFunction :> (FractionBox[#1, #2] &), 
        InterpretationFunction :> (#3 &), 
        Tooltip -> Automatic]]

```

When functions appear outside of partial derivatives, they still appear as $v(t,x)$. Trying to fix this, I tried 

```
supressVariable[f_Symbol] := 
  f /: MakeBoxes[f[t, x], TraditionalForm] :=
    InterpretationBox[ToBoxes[f], f[t, x]]
SetAttributes[supressVariable, Listable]
supressVariable[{v, ρ, p, f}];

```

This works fine for both 

```
f[t, x] v[t, x] == 0 // TraditionalForm

```

and 

```
ρ[t, x]*Derivative[0, 1][v][t, x] + 
   v[t, x]*Derivative[0, 1][ρ][t, x] + 
   Derivative[1, 0][ρ][t, x] == 0 // TraditionalForm

```

producing nicely readable equations. However, the simple 

```
f[t, x] == 0 // TraditionalForm

```

gives error message 

> 
> An unknown box name (ToBoxes) was sent as the BoxForm for the expression. Check the format rules for the expression
> 
> 
> 

and I don't know what to do with this. Can anybody help me out?