BUG: Solving the Stokes equations defined with TrialFunction returns an error about firedrake

[diego-hayashi]

One possible fix:

diff --git a/firedrake/variational_solver.py b/firedrake/variational_solver.py
index bba281bc1..430a9c311 100644
--- a/firedrake/variational_solver.py
+++ b/firedrake/variational_solver.py
@@ -366,7 +366,7 @@ class LinearVariationalProblem(NonlinearVariationalProblem):
         else:
             if not isinstance(L, (ufl.BaseForm, slate.slate.TensorBase)):
                 raise TypeError("Provided RHS is a '%s', not a Form or Slate Tensor" % type(L).__name__)
-            if len(L.arguments()) != 1:
+            if len(L.arguments()) > 1:
                 raise ValueError("Provided RHS is not a linear form")
             F = ufl_expr.action(J, u) - L