code: def test_euler_sineg():
      psi = Function("psi")
      t = Symbol("t")
      x = Symbol("x")
      L = (
          D(psi(t, x), t) ** 2 / 2
          - D(psi(t, x), x) ** 2 / 2
          + cos(psi(t, x))
      )
      assert euler(L, psi(t, x), [t, x]) == [
          Eq(
              -sin(psi(t, x)) - D(psi(t, x), t, t) + D(psi(t, x), x, x),
              0,
          )
      ]
  
file_path: sympy/sympy/calculus/tests/test_euler.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.trigonometric import cos, sin
  from sympy.testing.pytest import raises
  from sympy.calculus.euler import euler_equations
label_desc: Use image node_id 4 for calling a global function with example usage: test_euler_sineg() without return types