code: def test_pde_separate():
      x, y, z, t = symbols("x,y,z,t")
      F, T, X, Y, Z, u = map(Function, "FTXYZu")
  
      eq = Eq(D(u(x, t), x), D(u(x, t), t) * exp(u(x, t)))
      raises(
          ValueError,
          lambda: pde_separate(eq, u(x, t), [X(x), T(t)], "div"),
      )
  
file_path: sympy/sympy/solvers/tests/test_pde.py
imports: from sympy.core.function import Derivative, Function
  from sympy.core.relational import Eq
  from sympy.core.symbol import Symbol, symbols
  from sympy.functions.elementary.exponential import exp, log
  from sympy.functions.elementary.trigonometric import cos, sin
  from sympy.core import S
  from sympy.solvers.pde import pde_separate, pde_separate_add, pde_separate_mul, pdsolve, classify_pde, checkpdesol
  from sympy.testing.pytest import raises
label_desc: Use image node_id 2 for calling a global function with example usage: test_pde_separate() without return types